mid-13c., "attempt to clear (someone) from blame, find excuses for," from Old French escuser (12c., Modern French excuser) "apologize, make excuses; pardon, exonerate," from Latin excusare "excuse, apologize, make an excuse for, plead as an excuse; release from a charge; decline, refuse, excuse the refusal of" (source also of Spanish excusar, Italian scusare), from ex "out, away" (see ex-) + causa "accusation, legal action" (see cause (n.)).
Sense of "forgive, pardon, accept another's plea of excuse" is from early 14c. Meaning "to obtain exemption or release from an obligation or duty; beg to be excused" is from mid-14c. in English, as is the sense "defend (someone or something) as right." Sense of "serve as justification for" is from 1530s. Related: Excused; excusing. Excuse me as a mild apology or statement of polite disagreement is from c. 1600.