Green’s Dictionary of Slang

1. (US, also land crab) a horse, usu. second-rate.

1846		Durivage & Burnham Stray Subjects (1848) 31: The brown horse he bought for a fast crab, and is mighty good for a rush.
1852		C.A. Bristed Upper Ten Thousand 221: A tavern where the owners of ‘fast crabs’ were wont to repose, to water their horses.
1904		Wash. Post (DC) 14 Aug. 4/1: He had an old land crab of a chaser that hadn’t won a race [...] for a couple of years.
1907		B. Fisher A. Mutt in Blackbeard Compilation (1977) 3: They must have been feeding that crab of mine on birdseed and snails.
1908		Sporting Times 22 Feb. 1/4: You’re running against a lot o’ crabs — a good horse like you! Now, just for the sake of old times, show your heels to these plurry crawfish, will you?
1917		Van Loan ‘The Last Chance’ in Old Man Curry 122: Don’t you quit on me, you crab! [Ibid.] 256: I got the good colt and you got the crab.
1957		‘Nino Culotta’ They’re a Weird Mob (1958) 73: Wish ’e’d brought this crab in.
1984		W. Murray Tip on a Dead Crab 92: We had what you call a dead crab.

2. (US black gang) a derog. term for a member of the Crips, as used by a rival Blood; also attrib.

1991		L. Bing Do or Die (1992) x: ‘Yo — y’all want me to tell you how I kill crabs?’ (‘Crab’ is one of the insulting terms employed by Bloods to describe Crips).
1993		(con. 1978) K. Scott Monster (1994) 22: You little crab-ass punk!
2000		Bloods & Crips ‘Bangin’ on Wax’ 🎵 Fuck all these Crab niggas / Guess what, guess who, Crab killa’s back / [...] / So I’ll be rollin’ on these Crab niggas everynight / Crip Crip Crip, buck buck, time to go off.
2001		(con. 1990s) in J. Miller One of the Guys 157: ‘It was a Crab that raped my girl’.