excuse (v.)

c. 1200, "reason or motive for a decision, 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," which is of unknown origin.

From mid-14c. as "cause of an effect; source, origin." From late 14c. as "that which affords opportunity for a cause to operate, occasion;" also "reason for something taking place or for something being so; rational explanation." Also late 14c. as "proper or adequate reason, justification for an action." The sense of "matter of interest or concern; a side taken in controversy" is from c. 1300. Cause célèbre "celebrated legal case" is 1763, from French. Common cause "a shared object or aim" is by 1620s.

1743, "a plea of having been elsewhere when an action took place," from Latin alibi (adv.) "elsewhere, somewhere else," locative of alius "another, other, different" (from PIE root *al- (1) "beyond"). The weakened sense of "excuse" is attested since 1912, but technically any proof of innocence that doesn't involve being "elsewhere" is an excuse (n.) and not an alibi.