Green’s Dictionary of Slang

1. a prostitute.

1674

Head Canting Academy (2nd edn) 74: He is a Protector to all distressed Damsels called in our Vulgar tongue Common Whores, and that he may put a better gloss on the matter calleth them Cousons.

2. (US) a friend; usu. a term of address.

1876		in Harper’s Monthly. Dec. 21: ‘That’s so, cousin!’ exclaims a grim old Californian adventurer.
1943		R.L. Bellem ‘Dead Man’s Shakedown’ in Dan Turner Detective Mar. 🌐 Nudge this heap, cousin. Make knots to Sid Waldring’s home grounds.
1947		Berrey & Van den Bark Amer. Thes. Sl. (Supplement).
1953		L. Durst Jives of Dr. Hepcat (1989) 1: A group of fellows see a particular good dancer and are discussing him. ‘Jimson, you can believe that cat’s wings are not clipped because he is naturally buzzing cuzin.’.
1966		B. Jackson Get Your Ass in the Water (1974) 164: The Lion knew that he didn’t play the Dozens / and he knew that the Elephant wasn’t none of his cousins.
1976		Jenkins & Shrake Limo 234: Cooper said, ‘I think if I was you, cousin, I’d get on down to Ft. Lauderdale quick as I could’.
2014		Observer 13 Apr. 17/4: My cousins would tear apart McDonald’sl.

3. (gay) an older man’s younger lover.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 9: cousin (n.): The homosexual lover of another invert.
1972		B. Rodgers Queens’ Vernacular 55: cousin the hands-off lover of a gay friend.

4. (US police) one who gives the police regular payoffs.

1973		P. Maas Serpico 201: After the prisoner had been booked, there was a tense confrontation with [Officer] Stanard. [...] ‘What the fuck are you doing? You want collars, we’ll give them to you, any kind you want, but lay off the cousins’.
1996		(con. mid-1960s) J. Lardner Crusader 108: Gamblers who were on the pad were called ‘cousins’ .