why (adv.) Look up why at Dictionary.com
Old English hwi, instrumental case (indicating for what purpose or by what means) of hwæt (see what), from Proto-Germanic adverb *hwi (cognates: Old Saxon hwi, Old Norse hvi), from PIE *kwi- (source of Greek pei "where"), locative of *kwo- "who" (see who). As an interjection of surprise or emphasis, recorded from 1510s. As a noun, "cause, reason" from c.1300.