1. (adv.) For what cause, reason, or purpose; on what account; wherefore; -- used interrogatively. See the Note under What, pron., 1.
2. (adv.) For which; on account of which; -- used relatively.
3. (n.) The reason or cause for which; that on account of which; on what account; as, I know not why he left town so suddenly; -- used as a compound relative.