From PIE *kwos — the only English 'wh-' word where the 'w' is completely silent, pronounced /hoo/.
Asking which person or people; used to introduce a clause giving further information about a person.
From Old English "hwā" (who, anyone), from Proto-Germanic *hwaz, from PIE *kʷos/*kʷis (who, what), the interrogative pronoun stem. The PIE interrogative *kʷ- is one of the most securely reconstructed elements of the proto-language, surviving in virtually every branch: Latin "quis/quod" (who/what), Greek "tís/tí" (who/what, with regular sound change from *kʷ to t), Sanskrit "káḥ/kím" (who/what), Old Irish "cía" (who), Lithuanian "kàs" (who), and Hittite "kuiš" (who). The labio-velar *kʷ underwent different fates in each branch: in Germanic it became *hw- (preserved in English spelling "wh-"), in Greek it became "t-", in Latin "qu-", and in Celtic
'Who' is the only common English 'wh-' word where the 'w' is completely silent. 'What,' 'when,' 'where,' 'why,' 'which' all historically had /hw/ (and still do in some dialects), but 'who' is /huː/ — the /w/ vanished centuries ago. The 'w' in the spelling is purely historical, a fossil of the Old English 'hwā.'