We all know Puck as a character in A Midsummer Night's Dream, but 'puck' is actually a generic Middle English term for a fairy. Just as 'fairy' passed from French into other languages, so the English 'pouk/puck' went on a surprising etymological journey... (thread) 
‘Puck’ is cognate with Welsh ‘bwca’ and Irish ‘púca’, and it is unclear how it entered English. It may even be a Brittonic word that survived into English, although it is unattested before the 14th century (the Old English term is, of course, ælf)
At around the same time as 'puck' is first attested, it was already being replaced by Norman French 'fae' (later 'fairy'), and by the early modern period 'puck' generally referred specifically to a solitary domestic or household fairy
However, by this time 'puck' had already passed into German (it's unclear exactly how this happened...), where it referred to a 'domestic goblin' or Hausgeist, a supernatural creature who performs household tasks
The word seems to have followed German speakers eastwards until it passed into the Lithuanian language in the region around Klaipėda (known as Memel under German Prussian rule) as 'pūkys', a local name for the Lithuanian spirit Aitvaras
Aitvaras is a complex figure who bears little similarity to the English or German puck. Because the Lithuanians were probably still pagan or semi-pagan at the time they borrowed the word, Pūkys (like Aitvaras) was honoured much more like a deity than just a household spirit
The English and German puck is a comparatively homely and unthreatening character, unlike the Irish/Welsh/Cornish púca/bwca/bucca who is a monstrous 'bogey beast'. But by becoming identified with the fire-spirit Aitvaras, Pūkys is once again a threatening figure
As one writer has observed, words for supernatural beings always involve 'conjuring meaning out of thin air', and the inevitable semantic instability of such terms means that they can change meaning drastically as they pass into other languages...
Fairies are always elusive tricksters, even in semantics and etymology
