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.