Why
Old English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.
wiktionary
From Middle English why, from Old English hwȳ(“why”), from Proto-Germanic *hwī(“by what, how”), from Proto-Indo-European *kʷey, instrumental case of *kʷis(“who”), *kʷid(“what”).
Cognate with Old Saxon hwī(“why”), hwiu(“how; why”), Middle High German wiu(“how, why”), archaic Danish and Norwegian Bokmål hvi(“why”), Norwegian Nynorsk kvi(“why”), Swedish vi(“why”), Faroese and Icelandic hví(“why”), Latin quī(“why”), Ancient Greek πεῖ(peî, “where”). Compare Old English þȳ(“because, since, on that account, therefore, then”, literally “by that, for that”). See thy.
why (plural whies)
etymonline
why (adv.)
Old English hwi, instrumental case (indicating for what purpose or by what means) of hwæt (see what), from Proto-Germanic adverb *hwi (source also of Old Saxon hwi, Old Norse hvi), from PIE *kwi- (source of Greek pei "where"), locative of root *kwo-, stem of relative and interrogative pronouns. As an interjection of surprise or emphasis, recorded from 1510s. As a noun, "cause, reason" from c. 1300.
