Old neuter plural accusative case of quis, from Proto-Indo-European *kʷih₂. Corresponds both formally and functionally to Megara Ancient Greek σᾰ́ (sắ, “what?”), and functionally to ὅτι (hóti); this and other evidence point to the Greek origin of the use of quia as a subordinator, in contrast to quod where there's stronger evidence for a native development.