No language is perfect, and English is no exception. There are always words or expressions that cannot be rendered from one language to another. English supposedly has the largest vocabulary of any languages in the world (7x more words than French !), and well-educated people typically know less than 10% of them. There would be too many English words and nuances that do not exist in other languages (thousands in French, Italian or Japanese), but much less the other way round. Here are examples of the occasional French words that do not have an exact translation in English, or not in a single word.
Words with no specific English translation keeping the same nuance or connotation
patte : foot or leg of an animal (not claw). Used very informally or impolitely for humans.
gueule : mouth of an animal. It can also be used very informally or impolitely for humans, in which case the American words "gob" or "yap" could be used. It can also mean "face" with the same animal connotation. In that case the word "mug" exist in English.
bouffer : to eat, but normally used for animals or in a very informal (and rather impolite) way for humans. This word exists in German ("fressen") and in Japanese (食う "kuu").
gibier : "game", but only in the sense of "(meat of) wild animal killed by hunters".
tartine : "slice of bread" (no single word for that in English).
tartiner : to spread butter, jam, honey, cheese, etc. on a slice of bread
tartinière/planche à tartiner : board to spread butter/jam on one's slice of bread.
fromage à tartiner : soft cheese to spread on a slice of bread.
panade : baby meal composed of and fruits (typically a crushed banana, a grated apple and a squeezed orange) and crushed biscuit or "Cérélac" (of Nestlé).
cassonade : a kind of brown sugar that comes in light brown ("blond") and dark brown ("brown") version.
ravier: small dish, a few centimetre deep (e.g. for custard, chocolate mousse or olives)
écoeurant : the nearest translation is nauseating, but it is not necessarily used for food that makes one want to vomit. It is more common for food that is too sweet or too rich (e.g. in butter). Eating too much cake or chocolate can be écoeurant.
souriant : smiling/smiley, but not used in English in a sentence like "the staff should be more smiling" (le personnel devrait être plus souriant).
caricature : in the meaning of "satirical cartoon". A regular cartoon (or animation) is a dessin animé in French.
chaîne (hi-fi/stereo) : a hi-fi system, but not necessarily "hi-fi". A set, which typically includes an amplifier, a radio, CD, MD, and formerly also tapes and disk player. The componants aren't important as long as there is an amplifier, loudspeakers, with something else and it is a set.
téléspectateur : TV audience, TV viewers/listeners.
journal televisé : TV news, not in the sense of the 'news reported', but only the 'programme in itself presented by a newscaster'.
levé, debout : to be "up", but only with the meaning of "out of bed". English has state adjectives for asleep and awake, but no special term for "up/out of bed". French has two (debout can also mean "standing" or "on one's feet").
lunettes : "glasses", "eyeglasses". The English term is particularily confusing when we say things like "replace the glasses of the glasses" (in French rempacer les verres des lunettes).
montures : "frames of glasses".
superficie : translates as "acreage" (but not just in acres), "(land) area" (of a building, room or field). No unique term for that in English.
jante : "rim of a (car) wheel".
dejanter : verb meaning that the tyre is getting out of the rim of the wheel (used mostly in car racing).
Germain(s) (Germani in Italian, Germanos in Spanish, Germanen in Dutch and German) : means "German(s)", but as the noun of "Germanic" (germanique in French, germanico in Italian and Spanish, germanisch in German), not the inhabitants of the country Germany (Allemands in French, Tedeschi in Italian, Alemanes in Spanish, Deutchen in German).
Several words in French with only one common translation in English (lost nuance) :
cheveux, poil, pelage : all translates as "hair". cheveux is only used for the hair on a human head. Body hair are called poils. Most mammals have a pelage, but individuals hairs are also called poil(s).
bouclé, frisé, crépu : all translate as "frizzy" or "curly" in English. However crépu means extremely frizzy like Black Africans for which there is no word in English.
bille, boule, boulet, balle, ballon, bal : there are only two words for this in English, "ball" and "balloon". In French, a ballon is a "balloon" or a "ball" with air inside. A balle is a ball with air inside, usually used for sports/games like ping-pong, tennis, football, volleyball, etc. A boule is not hollow, like the ball of bowling, billiard/snooker, or even a snowball, hairball, fireball, etc. A bille is like a boule, but smaller, like marbles, mercury balls, or balls used in ball-bearing mechanisms. A boulet is a "cannon ball" or the ball in "ball and chain". Bal is the word for a "dancing ball" (party).
capsule, capuchon, bouchon, casquette, calotte : there is only the word "cap" in English for all these. A capsule is a beer bottle cap, a capuchon is a plastic bottle/recipient cap, a bouchon is the general word for a recipient cap, a casquette is a cap to wear (e.g. baseball cap), a calotte is an "ice cap".
gel, gelée, givre : all mean "frost". Gelée and gel are used in meteorology (tempetures below freezing), while givre is the forst formation on (car) windows or ice crystals.
glace, glaçon, glaciaire, glacé, glacier, glacial, verglas : all translate as "ice", either as noun or adjective. Glace is the most general term. A glaçon is an ice cube (e.g. for drinks). Verglas is ice on roads after a wet night in freezing temperature. A glacier is either a "glacier" (in geology) or an "ice-cream maker" (the person or company). Glaciaire is the noun for a "machine to make ice-cream" or the adjective referring to the "ice age" (période glaciaire), "ice cap" (calotte galciaire). Glacial can be used like the English word "glacial", or to mean "icily" or "freezing cold". Glacé means "frozen" or "ice" like in "ice-cream" (crème glacée).
fenêtre, carreau, vitre, vitrine, vitrail : there are only two words in English for this, "window" and "pane". A fenêtre is a window with the frame and glass. A carreau is just one pane, especially when the window is divided in many panes like in a "stained glass window" (vitrail, in French). A vitre is a pane, usually when there is only one for the whole window. A vitrine is a large window pane, usually in shops to display the products.
mur, muraille, cloison, paroi : all translate as "wall". The difference is that a mur is made of masonry (brick, stone or concrete blocks), while a cloison is hollow and just made of panels and plaster. A paroi is also an "inner wall" usually made of wooden panels, but the word is also used for the outer walls of a wooden fortification (e.g. Roman fort). A muraille is a defensive, fortified wall, normally made of masonry, like medieval city walls or even the Great Wall of China.
table, tableau : both are "tables" in English. A table is the piece of furniture (to eat), while a tableau is a framed painting or a graphic table (with colums and rows).
encadrement, cadre, châssis, armature, charpente : all translate as "frame". An encardement is a picture frame, a cadre is a picture or bed frame, a châssis is a window frame, an armature is wooden or metalic frame serving as support to build something, and a charpente is a roof frame.
tuile, carrelage : both translate as "tile". Tuile means "roofing tile", while carrelage refers to "flooring tiles".
lavette, torchon, chiffon, serpillière, vêtement(s) : all translate as "cloth" in English. The first two mean "dishcloth" (torchon can also mean floorcloth), the third is a rag (old cloth) or cloth for cleaning (housecloth), the fourth is a floorcloth, and the last are clothes to wear.
coupe, tasse : Coupe means "cup" but only in the meaning of trophy (e.g. Coupe du Monde => World Cup). A cup to drink is a tasse in French (the English word "mug" has also become common recently for mugs, which used to be called "tasse" as well).
gourmand, gourmandise : "gourmand" in English as well. However, there is no noun for gourmandise ("greed" is avidité in French).
doux, mou : both translate as "soft" in English, although their meaning is very different. Doux is the opposite of "rough" or "coarse" (rugueux), while mou is the opposite of "hard". Doux can also mean sweet, but almost only for wines (otherwise sucré is used).
bricolage : do-it-yourself
bricoler : verb meaning to "fiddle", "putter" or "tinker", used for do-it-yourself home repair, amateur carpentery, plumbing, electricity, mounting furniture bought in detached pieces, etc.
bricoleur : person doing bricolage ("handyman").
batterie, pile (バッテリ vs 電池 in Japanese) : both translate as "battery" in English. The first one is typically rechargeable (e.g. for mobile phones, camcorders, car engines...), while the 2nd one is usually discardable, low voltage (1.5 to 9 volts) and not product specific (can be used for all can of electronic appliances).
station, gare : both translate as "station" in English. The former is used only for metro/subway/underground, while the latter is for trains or buses.
boucherie, charcuterie : both translate as butcher's shop, although the former specialises in meat, and the latter in cooked meats (sausages, pâté, etc.)
travail, travaux, oeuvre, ouvrage : all translate as "work"; "travail" is the general sense (working), "travaux" is usually used for "construction works", "oeuvre" means 'work of art', and "ouvrage" usually means "work" in the sense of dutiful, hard or bothersome work.
travailleur, ouvrier : "worker", the former in the general sense (someone who works), the latter in the sense of 'manual worker' (in a factory, in construction, etc.)
docteur, médecin : the former means "doctor" in the general sense (PhD), while the latter only means "medical doctor".
mendiant, gueux : both translate as "beggar" in English, yet the meanings are completely different in French. A mendiant is someone who begs (for money), while a gueux is a person who does not belong to the nobility.
gens, peuple, peuplade : all translate as "people". The first one is the people in general (e.g. people say that...), although it cannot be used simply as the plural of person (gens is uncountable, so never say 3 gens, but 3 personnes). The second one refers to an ethnic, cultural or national group. The third carries the meaning of ethnic group too, but at a subnational level, usually a tribe, nomadic group, or minority ethnicity linked to one particular region.
hibou, chouette : both are "owls" in English, but it isn't the same bird. hiboux have external ears, while chouettes don't.
tiers, troisième : Both mean "third", but tiers is "third" between "half" and "quarter", while troisième is "third" between "second" and "fourth". As a noun, tiers can also mean a third party or a third person.
crue, inondation, flot : all translate as "flood". A crue is when the level of a river rises due to seasonal changes (e.g. the Nile). An inondation is a flood in general (e.g. due to a broken piping, a bathtub that overflows, too much rain, etc.). The term flot is only used in the metaphorical sense (e.g. flood of words, flood of tears, flood of tourists...). Flux can also be used in the sense, although it generally translates as "flow".
nocif, nuisible, néfaste : all mean "harmful", but each is used in a different context. Nocif means "harmful for health" (usually of products or a particular environment) but not only by being toxic (e.g. can be used for a video game addiction). Nuisible is normally used to describe the harmful behaviour of a life being on others (e.g. organismes nuisibles). Néfaste is used to talk about the harmful effects of something, on health, society, business or whatever.
sens unique, sens interdit : both refer to a "one-way street", but each in an opposite direction. In the right direction, one takes a sens unique, while in the wrong direction it is a sens interdit.
Words with many approximate or specific translations in English but no general term keeping the overall meaning
abonnement : means "subscription", "season ticket", "weekly/monthly/yearly ticket/subscription", etc. Very useful as it can be used for anything : magazines/newspaper/satellite TV subscriptions, public transport tickets, yearly cinema/theatre ticket/card, fitness subscriptions, (mobile) phone monthly fee, mailing list subscriptions, etc. People just say the have an "abonnement" without having to specify the length or type. It doesn't matter whether it's a ticket, a card, a pass, or none of these - it works in any context.
devis : "estimate", "quotation", "tender" for a particular work (especially in construction and maintenance). There is no unique word with only that meaning in English.
chantier : construction site, workings, yard...
empêchement : unexpected last minute change of plans/schedule, unforeseen difficulty, unexpected obstacle, hitch, impediment, "something cropped up"... (French speakers very commonly explain that they cannot come to an appointment because of an empêchement - it is a very convenient word with no real translation in English)
parcours : a track, journey or run, but with a demarcated route, and often also with stops, stands or events along the way. It is also used in expressions like "round of golf", "assault course" or "adventure playground". It is difficult to translate because it can be used in any context. For instance some TV games have parcours with obstacles or épreuves (see below).
épreuve : can be translated as test, exam, ordeal, obstacle, sports event, trial, proof... depending on the context. It comes from prouver (to prove), so it is a test or trial to prove one's abilities, be it intellectual, physical, moral or psychological.
sportif : a person that does sport. The words "athlete" or "players" have similar meanings but can only be used for specific sports (e.g. you can't say a ski player, or a tennis athlete). There is no word in English that can be used to describe a person practising any sport or physical activity, from a runner to the climber, F1 driver, football player, swimmer, etc. So you can't ask in English "Are you a sportif", as this word doesn't exist. You can say "Do you do sports ?", but it doesn't mean the same. A sportif is someone who does a lot of sports or likes practising sports, not just anyone that does it. There is the word "sportsman", which is the nearest equivalent, but is not much used, and is not gender neutral.
stage, stagiaire : these words mean respectiely "internship" and "intern" (in US English), but also "training course", "vocational training (course)", "work placement", "work experience scheme", etc., and the person who does it (the stagiaire).
accueil : means "welcome", "reception", "home page" (of a website), "act of welcoming", or even "quality of welcome" (from a hotel, restaurant, club or company staff).
pique : "something with a pointed end", "something that prickles or stings", "spades" (in playing cards), "barb"...
manquer : to lack, to miss, to default, to be short of, not be enough, to be insufficient, to run out of... No general term for all those meanings combined in English.
supplier : depending on the context, it translates as "to supplicate", "to beg", 'to plead", "to entreat" or "to adjure". "Supplicate" is the nearest translation, but used mainly when addressing God in English, while it has a daily usage (like "to beg") in French. E.g. to a child : Je t'en supplie, arrête de pleurnicher ! (literally "I "supplicate you", stop whining !")
escroc : con artist, conman, crook, huckster, swindler, twister, slicker, shark, sharper, sharpie, hood, hoodlum, hustler...
escroquerie : fraud, scam, cheating, swindling, swindle, rip off, humbug, setup, hoax, false pretences...
Unique words in French with compound translations in English
In this category, the French words have an English equivalent with the exact same meaning. However, the English term is always a compound of 2 or 3 words or more, which can be clumsy and unpractical in many situations. For example, Internet users like keywords in one block. It is easier to search for a mitigeur than a "single handle faucet". Website owners prefer to have a domain name without - (hyphen) or _ (underscore) in it. The website of a shop in Montreal that specialises in wall-to-wall carpet (as opposed to Oriental carpets or rugs) will find it much more convenient to have a domain like montreal-moquette.ca than montreal-wall-to-wall_carpet.ca. Likewise, your city's local ice skating ring will prefer to have a single word like patinoire than a three-word compound, be it for the domain name, site title or search keywords.
immobilier : "real estate" ("housing" and "lodging" translate as logement in French, and "property" as propriété ; the meanings are similar but can't always be used instead of "real estate")
déménager, déménagement : "to move house" (verb) and "house moving" (noun).
emménager, emménagement : "to move in" and the "action of moving in" (with someone or in a new house).
déménageur : house mover, furniture (re)mover.
électroménager : home electric appliances (家電 in Japanese), i.e. everything from audio-video to kitchen electric appliances to computers and mobile phones.
télédistribution : "television broadcasting by cable"
chaudière : "gas boiler" or "oil furnace"
cuisinière : cooking range
hotte : "exhaust hood", "cooker hood" (for the kitchen). No unique word for that in English.
friteuse : deep fryer
mitigeur : "mixing valves", "single handle faucet/tap".
aspirateur : vaccum cleaner
aspirer : "to use a vaccum cleaner" ("to suck dust")
moquette : wall-to-wall carpet (as opposed to tapis, which is a regular carpet)
chambranle : door frame
sommier : (slatted/spring) mattress/bed base (as opposed to matelas which is the mattress itself, not the support).
taie (d'oreiller) : pillow case ("case" has many possible meanings in English, but taie in French only means "pillow case")
dressing : "walk-in closet" (a regular closet is a placard, while a wardrobe is a garde-robe).
finitions : finishing touches
bottin : phone book
consigne : "return for refund" (bottle, glass)
biberon : baby bottle
arrosoir : water can (for plants)
loupe : magnifying glass
levure : baking powder
purée : mashed potatoes
tisane : herbal tea
arête : fishbone (it is really just another compound, "fish + bone")
vestiaire : locker room
piscine : swimming pool
patinoire : ice skating ring
patisserie : pastry shop
friterie : "chip shop" in BrE, but no particular word in AmE (not really the same as "hamburger stand" as many friteries don't sell hamburgers at all)
amortisseur : shock absorber
détour : roundabout way
chevreuil : roe deer (a deer is a cerf in French)
sanglier : wild boar (a regular boar is a verrat in French)
marcassin : young wild boar (a piglet is a goret in French)
cachalot : sperm whale (a whale is either a baleine or a rorqual in French)
orque : killer whale
globicéphale : pilot whale
Verbs and adjectives
exaucer : to grant/fulfill a wish
saouler : to get drunk
saoulant : making one get drunk (or "tiring" in slang)
frime, frimer, frimeur : show-off (English does not have different words for the noun, verb and person noun)
borgne : one-eyed (person)
frileux : sensitive to the cold
malencontreux : adjective including the meanings of "ill-timed", "untimely", "inopportune" and "unfortunate" all at the same time.
loisible : adjective meaning "to have the liberty to do something if and whener one wishes to do it". Includes the meaning of permission and doing it at one's leisure or discretion.
cernes : (dark) rings/circles under/around the eyes
minuscule >< majuscule : small font >< capital letter
devise : foreign currency, hard currency
brelan : three of a kind (in poker)
carré : four of a kind (in poker)
aller (noun) : "outward journey", "way to go", or "one way/single ticket". Aller is the opposite of retour ("return"). It is difficult to express this kind of sentence in English : Lequel a été le plus fatiguant, l'aller ou le retour ?" ("which one has been the more tiring, the outward journey or the return journey ?"). It doesn't sound very natural in English, although it is very common in French.
débouché : job opportunities (used a lot in reference to university studies)
dénivellation : difference in height/level/altitude.
pépinière : tree nursery
clocher : bell tower
intemperies : bad weather
mélomane : music lover
dorénavant : from now on, from then on, from this point forward (there is actually a one-word translation in English, but hardly ever used nowdays : "henceforth" or "henceforward")
armoiries, blason, emblème, écu : The armoiries refer to the full "coat of arms" of a person or family, while the blason or écu is only the central part of the arms (in the shape of a shield) without the motto and decoration around.
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