2、ex-, 向外。-cus,原因，词源同causal, because.
2、excuse:  Etymologically, excuse means ‘free of accusation’. It comes via Old French from Latin excūsāre, a compound verb formed from the prefix ex-, denoting removal, and causa ‘cause’ – but ‘cause’ in the sense not of something that produces a result, but of ‘legal action, accusation’ (a meaning preserved in English ‘cause list’, for example) Originally, the s of both the noun and the verb was pronounced /z/; the /s/ of the modern English noun arose by analogy with such nouns as use and abuse.=> accuse, cause
4、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.
6、late 14c.,"pretext, justification,"from Old French excuse, from excuser (see excuse (v.)). The sense of"that which serves as a reason for being excused"is recorded from mid-15c. As a noun, excusation is the earlier form (mid-14c.).