from out of nowhere; from who knows where
Describes something appearing, arriving, or being heard without a clear source. Often used with verbs like 現れる, 聞こえる, or やって来る.
A cat appeared from out of nowhere.
Music could be heard from who knows where.
Full kanji spelling with 無く; extremely rare and archaic-looking.
A fixed expression composed of どこ (where), から (from), と (quotative particle), and なく (adverbial form of ない, 'not exist'). Literally 'without [saying] from where', it has become a set adverbial phrase meaning 'from an unknown place'.