The Irish word fuinneog /ˈfɪnʲːoːg/ ‘window (fem.)’—uinneig in Scottish Gaelic; fuinneoig in the Irish of Cois Fharraige where the dative singular has been assumed as the dictionary form for ā-stem nouns—may appear obscure in origin. However, it has a neat etymology as an old loanword from Germanic.
The word is from Old Norse vindauga ‘window (neut.),’ a composition of vindr ‘wind’ and auga ‘eye’—that is, figuratively, a sort of eye through which the wind enters through. This word was also loaned into English as window; *wind-augō may have been a formation specific to North Germanic, since both Old English ēag-duru and Gothic áuga-daúrō reflect the composition ‘eye-door.’
The ending –eog (the equivalent of –óg when following a palatalized consonant) and the feminine gender that accompanies it is on analogy to other nouns in -óg—cf. féasóg ‘beard.’ It is a frequent additive to loanwords—cf. spúnóg ‘spoon,’ from English spoon.
We can surmise from orthographic tendencies in manuscripts that nasal+stop clusters (*mb, *nd, etc.) were becoming geminate nasals (*mm, *nn, etc.) broadly over the course of the Old Irish period. The central consonant in modern fuinneog would suggest that vindauga was loaned before or during this transitional period. Old Norse v would have had the pronunciation /w/ (hence the English pronunciation), allowing the first consonant of the ensuing Irish word to fit neatly into the extant mutational paradigms of inherited f < *w (although the initial f– was later lost in Scottish Gaelic on analogy to words beginning with Ø-; the opposite happened in fosgail ‘open,’ with f– springing into existence ex nihilo; cf. Irish oscail).