mother-in-law (n.)

"female parent, a woman in relation to her child," Middle English moder, from Old English modor, from Proto-Germanic *mōdēr (source also of Old Saxon modar, Old Frisian moder, Old Norse moðir, Danish moder, Dutch moeder, Old High German muoter, German Mutter), from PIE *mater- "mother" (source also of Latin māter, Old Irish mathir, Lithuanian motė, Sanskrit matar-, Greek mētēr, Old Church Slavonic mati), "[b]ased ultimately on the baby-talk form *mā- (2); with the kinship term suffix *-ter-" [Watkins]. Spelling with -th- dates from early 16c., though that pronunciation is probably older (see father (n.)).

Sense of "that which has given birth to anything" is from late Old English; as a familiar term of address to an elderly woman, especially of the lower class, by c. 1200.

Mother Nature as a personification is attested from c. 1600; mother earth as an expression of the earth as the giver of life is from 1580s. Mother tongue "one's native language" is attested from late 14c. Mother country "a country in relation to its colonies" is from 1580s. Mother-love "such affection as is shown by a mother" is by 1854. Mother-wit "native wit, common sense" is from mid-15c.

Mother of all ________ (1991), is Gulf War slang, from Saddam Hussein's use in reference to the coming battle; it is an Arabic idiom (as well as an English one), for instance Ayesha, second wife of Muhammad, is known as Mother of Believers; the figure is attested in English in 19c. (Virginia is called mother of commonwealths from 1849). Mother Carey's chickens is late 18c. sailors' nickname for storm petrels, or for snowflakes.

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 formation is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited. Thus the word originally had a more narrow application; its general extension to more distant relatives of one's spouse is, according to OED "recent colloquial or journalistic phraseology." Middle English inlaue (13c.) meant "one within or restored to the protection and benefit of the law" (opposite of an outlaw), from a verb inlauen, from Old English inlagian "reverse sentence of outlawry."