c.1200, "reason for action, grounds for action; motive," from Old French cause "cause, reason; lawsuit, case in law" (12c.), and directly from Latin causa "a cause; a reason; interest; judicial process, lawsuit," of unknown origin.
In English, sense of "matter of concern; side taken in controversy" is from c.1300; that of "the source of an effect" is early 14c.; meaning "reason for something taking place" is late 14c. Cause célèbre "celebrated legal case" is 1763, from French. Cause why? "for what reason?" is in Chaucer.