ad. For what purpose, reason, or cause; with what intention, justification, or motive: Why is the door shut? Why do birds sing? conjunction. The reason, cause, or purpose for which: I know why you left. conjunction. 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. The cause or intention underlying a given action or situation: studying the whys of antisocial behavior. n. A difficult problem or question.interjection. Used to express mild surprise, indignation, or impatience.