whyyoudaoicibaDictYouDict
why: [OE] Why goes back to Indo-European *qwei, the locative case of the interrogative base *qwo- (source of English what and who). This passed into prehistoric Germanic as *khwī, which has since died out in all the Germanic languages apart from Danish (hvi) and English (why).
why (adv.)youdaoicibaDictYouDict
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.