What's the "een" in Halloween?
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.
late 14c., "produce an effect," also "impel, compel," from Old French causer "to cause" (13c.) and directly from Medieval Latin causare, from Latin causa "a cause; a reason; interest; judicial process, lawsuit," of unknown origin. Related: Caused; causing. Classical Latin causari meant "to plead, to debate a question."