It seems, that exactly the same name is used by Latvians in form of Jānis. By exactly I mean sounds the same.
Why I'm thinking, that there might be some connection, because there already exists proven Greek-Baltic connections. For example some of the hills, that were used for gathering of folk were called Dāmu kalns(Hill of Lady), but they had nothing to do with Madame, but more probably with gathering of Deimos and name was corrupted over times - from dēm to dām. There exist some idea, that initial Baltic people traveled from south(around Dacian region) in ancient times(this theory so far makes sense), that precedes Hellenistic(IE) Greeks - at least that's where comparison of archeological teeth comes into.
Latvians claims Jānis to be theirs.
Up till arrival of (west)christians Jānis was not a birth name. Jānis was only a given name to a man, who had children. Actually I'm wrong - it became given birth name to Latvians only 4-6 centuries ago.
In modern times there are celebrations in Solstice(now shifted, because of calendar) of Jānis and it is regarded as god of fertility and prosperity.
The only etymology I could dig up is that there were Jēnes, that had something to do as a gates to Solar(usually - also megalithic, because stones was used - considering, that there are not that many large stones available in Baltic) calendar. I suppose, that some of those gates were erected during solstice and this transferred over times to a person who was responsible for calendar. Jēnes name was preserved only as house names - around sacred hills that stood on lines of Solstice geometry - lines that connected top of the sacred hill with rising sun during solstice. Actually there were other houses that stood on other lines, that connected rising of Sun in other dates, but Jēne stood on the line, that connected it with Summer Solstice only.
Idea about gates and calendar connects it to ancient Romanian god of gates, Janus:
https://en.wikipedia.org/wiki/Janus
The problem of Latvian only connection about Jānis is that Lithuanians does not have any idea of paganic Jānis. the problem is that Latvians and Lithuanians were same folk initially. There are two explanations for this - either Lithuanians initially knew about this deity, but they accepted christianity much earlier and forget or Latvians acquired it from christians. And regional Lithuanian varieties who know about this deity has Latvian variation. The problem about acquisition from christians is that Letts erected hillforts and connected them with Jēnes, is that latest hillforts were erected at least 100 years before early christianity reached them in 6th century and they did not reached them where the latest hillforts were erected.
Another problem is that Slavic people have Ivan, Belorussian even have diminutative form Janka, but they seems to have received it from christianity or have erased other memories. Also because of christianity, especially if Slavic for most Slavic people initially was not native, but church language, so it is useless to search any ideas there. Yes, there exist paganic celebrations of Ivan Kupala, but this really seems like something that reverted from John the Baptist, especially as both have idea of dipping in water.
Latvians had many religious terms acquired from early east christians - svēts(sacred) comes from Slavic, there were some other religious terms, so it might be so, that even Jānis comes from them, too. There seems to be Finnish variation of the name, but not really sure how ancient is it and if this is actually borrowed from Germans, so I will skip it.
It might be paganic, but then it might be also one of those synergetic deities, that christianity didn't tried to erase, but adopted and made their own, so in some cases it reintroduced it among pagans to make Solstice day christian.
Anyway - is there Greek etymology or anything that explains origin of this word of nonchristian origin?
Wiki explanation is not satisfactory, because Bible can't be source for all names, especially if it doesn't want to touch other gods, like Janus...
Why I'm thinking, that there might be some connection, because there already exists proven Greek-Baltic connections. For example some of the hills, that were used for gathering of folk were called Dāmu kalns(Hill of Lady), but they had nothing to do with Madame, but more probably with gathering of Deimos and name was corrupted over times - from dēm to dām. There exist some idea, that initial Baltic people traveled from south(around Dacian region) in ancient times(this theory so far makes sense), that precedes Hellenistic(IE) Greeks - at least that's where comparison of archeological teeth comes into.
Latvians claims Jānis to be theirs.
Up till arrival of (west)christians Jānis was not a birth name. Jānis was only a given name to a man, who had children. Actually I'm wrong - it became given birth name to Latvians only 4-6 centuries ago.
In modern times there are celebrations in Solstice(now shifted, because of calendar) of Jānis and it is regarded as god of fertility and prosperity.
The only etymology I could dig up is that there were Jēnes, that had something to do as a gates to Solar(usually - also megalithic, because stones was used - considering, that there are not that many large stones available in Baltic) calendar. I suppose, that some of those gates were erected during solstice and this transferred over times to a person who was responsible for calendar. Jēnes name was preserved only as house names - around sacred hills that stood on lines of Solstice geometry - lines that connected top of the sacred hill with rising sun during solstice. Actually there were other houses that stood on other lines, that connected rising of Sun in other dates, but Jēne stood on the line, that connected it with Summer Solstice only.
Idea about gates and calendar connects it to ancient Romanian god of gates, Janus:
https://en.wikipedia.org/wiki/Janus
The problem of Latvian only connection about Jānis is that Lithuanians does not have any idea of paganic Jānis. the problem is that Latvians and Lithuanians were same folk initially. There are two explanations for this - either Lithuanians initially knew about this deity, but they accepted christianity much earlier and forget or Latvians acquired it from christians. And regional Lithuanian varieties who know about this deity has Latvian variation. The problem about acquisition from christians is that Letts erected hillforts and connected them with Jēnes, is that latest hillforts were erected at least 100 years before early christianity reached them in 6th century and they did not reached them where the latest hillforts were erected.
Another problem is that Slavic people have Ivan, Belorussian even have diminutative form Janka, but they seems to have received it from christianity or have erased other memories. Also because of christianity, especially if Slavic for most Slavic people initially was not native, but church language, so it is useless to search any ideas there. Yes, there exist paganic celebrations of Ivan Kupala, but this really seems like something that reverted from John the Baptist, especially as both have idea of dipping in water.
Latvians had many religious terms acquired from early east christians - svēts(sacred) comes from Slavic, there were some other religious terms, so it might be so, that even Jānis comes from them, too. There seems to be Finnish variation of the name, but not really sure how ancient is it and if this is actually borrowed from Germans, so I will skip it.
It might be paganic, but then it might be also one of those synergetic deities, that christianity didn't tried to erase, but adopted and made their own, so in some cases it reintroduced it among pagans to make Solstice day christian.
Anyway - is there Greek etymology or anything that explains origin of this word of nonchristian origin?
Wiki explanation is not satisfactory, because Bible can't be source for all names, especially if it doesn't want to touch other gods, like Janus...