英语词源字典 - 在线词源网站

beldam (n.)[beldam 词源字典]: "aged woman," 1570s; earlier "grandmother" (mid-15c.), from dame (q.v.) in the sense of "mother" + bel-, Middle English prefix expressing relationship (as in belfader, belsire "grandfather"), from Old French bel, belle "beautiful, fair, fine" (see belle). This "direct relationship" sense of bel is not found in French, where the prefix is used to form words for in-laws.[beldam etymology, beldam origin, 英语词源]
in-law (n.): 1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.
The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. ["Blackwood's Magazine," 1894]
The earliest recorded use of the phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.