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.