1. For what cause, reason, or purpose; on what account; wherefore; used interrogatively. See the note under What, pron. Turn ye, turn ye from your evil ways; for why will ye die, O house of Israel? (Ezek. Xxxiii. 11)
2. For which; on account of which; used relatively. No ground of enmity between us known Why he should mean me ill or seek to harm. (milton) Turn the discourse; I have a reason why I would not have you speak so tenderly. (Dryden)
Why is sometimes used as an interjection or an expletive in expression of surprise or content at a turn of affairs; used also in calling. Why, Jessica! If her chill heart I can not move, Why, I'll enjoy the very love. (Cowley) Sometimes, also, it is used as a noun. The how and the why and the where. (goldsmith) For why, because; why. See Forwhy.
Origin: OE. Whi, why, AS. Hwi, hw, instrumental case of hwa, hwaet; akin to Icel. Hvi why, Dan. & Sw. Hvi; cf. Goth. Hw. See Who.