son-in-law n.

"male child in relation to either or both parents," Old English sunu "son, male child," also broadly "male descendant;" also "second person of the Trinity," from Proto-Germanic *sunus (source also of Old Saxon and Old Frisian sunu, Old Norse sonr, Danish søn, Swedish son, Middle Dutch sone, Dutch zoon, Old High German sunu, German Sohn, Gothic sunus "son").

The Germanic words are from PIE *su(H)nus "son" (source also of Sanskrit sunus, Greek huios, Avestan hunush, Armenian ustr, Lithuanian sūnus, Old Church Slavonic synu, Russian and Polish syn "son"), a derived noun from root *su(H)- "to give birth" (source also of Sanskrit sauti "gives birth," Old Irish suth "birth, offspring").

The sense of "person whose character partakes so much of some quality as to suggest the relation of a son to a parent" was in Old English. As "person regarded as the product of some place," 1590s.

Son of _____ as the title of a sequel to a book or movie is recorded from 1917 ("Son of Tarzan"). Most explanations for son of a gun (1708) are more than a century after its appearance. Henley (1903) describes it as meaning originally "a soldier's bastard;" Smyth's "Sailor's Word-Book" (1867) describes it as "An epithet conveying contempt in a slight degree, and originally applied to boys born afloat, when women were permitted to accompany their husbands to sea ...."

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."