quid pro quoyoudaoicibaDictYouDict[quid pro quo 词源字典]
1560s, from Latin, literally "something for something, one thing for another," from nominative and ablative neuter singulars of relative pronoun qui "who" (see who) + pro "for" (see pro-) + quo, ablative of quid.[quid pro quo etymology, quid pro quo origin, 英语词源]