adv. For what purpose, reason, or cause; with what intention, justification, or motive: Why is the door shut? Why do birds sing?
conj.
1. The reason, cause, or purpose for which:
2. Usage Problem. On account of which; for which: "The reason why [regular verbs] are called regular is that we can predict what all the other three forms are" (Randolph Quirk).
n., pl., whys.
1. The cause or intention underlying a given action or situation: studying the whys of antisocial behavior. 2. A difficult problem or question.
interj. Used to express mild surprise, indignation, or impatience.
