"woe!;" (Spain) "great!"
13th cent. There are three origin theories of this word. If a majority of linguists is to be taken as some sort of consensus, then guay is probably from Latin væ "woe." Alternatively, the word is from Gothic wai "woe."
From Proto-Germanic *wai 'id.' Both Latin væ and Proto-Germanic *wai trace back to Proto-Indo-European *u̯ai̯ 'id.' A third idea is that the word is from Andalusian Arabic *way. See Dworkin (2012) for further discussion.
Old Spanish guayar "to wail," guaya "wailing"
Italian guai, Romanian çai
Old Irish fáe "alas," Middle Welsh gwae 'id.,' Middle Breton goa'id.'
Old Norse vei "woe," Old High German wē "woe," Old Saxon wē 'id.,' Old English wā (English woe)
Avestan vaii-ōi (?)
The Spanish sense of "great" is a secondary formation through meaning reversal.