Origin and history of why

interrogative pronoun, Old English hwæt, referring to things in abstraction; also "why, wherefore; indeed, surely, truly," from Proto-Germanic pronoun *hwat (source also of Old Saxon hwat, Old Norse hvat, Danish hvad, Old Frisian hwet, Dutch wat, Old High German hwaz, German was, Gothic hva "what"), from PIE *kwod, neuter singular of *kwos "who," from PIE root *kwo-, stem of relative and interrogative pronouns. Corresponding to Latin quid.

As an adjective and adverb in Old English, lending interrogative force. As a conjunction in late Old English. Exclamatory use, emphasizing the speaker's emotional response, calling attention, or introducing a narrative, was in Old English. It is the first word in "Beowulf."

What, ho! as an exclamation or summons is by late 14c. What the _____ (devil, etc.) as an exclamation of surprise is from c. 1400. As an interrogative expletive at the end of sentences by 1785; common in affected British speech. As meaning "what did you say?" from c. 1300.

Or what as an alternative end to a question is attested by 1766. What have you "anything else one can think of" is from 1925. What's up? "what is happening?" first recorded 1881.

"To give one what for is to respond to his remonstrant what for? by further assault" [Weekley]. The phrase is attested from 1873; what for? "for what reason" is from late 14c., as framing a question, by 1760.

To know what is what "understand the qualities or natures of things" is from c. 1400; I'll tell you what to emphasize what is about to be said is in Shakespeare. Middle English also had an interjection whatkinnes "what? what sort of?"

"for whatever reason," by 1856, from why + ever.