mother-in-law (n.)youdaoicibaDictYouDict[mother-in-law 词源字典]
mid-15c., "mother of one's spouse," from mother (n.1) + in-law. Also in early use, "stepmother." In British slang c. 1884, mother-in-law was "a mixture of ales old and bitter."[mother-in-law etymology, mother-in-law origin, 英语词源]