Green’s Dictionary of Slang

1. in the game of hazard, the lowest throw, a pair of aces; thus fig. in phrs. below.

1768		Carlisle in Jesse George Selwyn (1843) II 238: I hope you have left off hazard. If you are still so foolish, and will play, the best thing I can wish you is, that you may win and never throw crabs.
1771		R. King New London Spy 116: Prompted by good fortune, he sets a large sum — crabbs! observe how old Gripus changes [...] the chop-fallen countenance.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions .
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1798		Sporting Mag. May XII 79/1: With these dice it is impossible to throw what is at Hazard called crabs; that is, aces, or ace and deuce.
1811		Lex. Balatronicum.
1825		C.M. Westmacott Eng. Spy I 179: What coming crabb over us, old fellow, hey? Very well, I shall bolt and try Randall.
1828		‘A Flat Enlightened’ Life in the West I 241: Aces and deuce ace are crabs to all the mains. Twelve is crabs to the mains five, seven, or nine; and eleven is crabs to five, six, eight, or nine. When crabs are thrown, the caster is out, and the box passes.
1832		Satirist (London) 22 Jan. 31/1: If [...] the caster throws aces, or two and one, he loses what he has staked [...] these two numbers being called crabs.
1834		(con. 1737–9) W.H. Ainsworth Rookwood (1857) 260: No crabs for me, I’m fly, d’ye see.
1840		R. Barham ‘The Black Mousquetaire’ in Ingoldsby Legends (1842) 2: We know in these cases / Your ‘Crabs’ and ‘Deuce Aces’ / Are wont to promote frequent changes of places.
1873		Sl. Dict. 132: Crabs in dicing, a pair of aces.
1874		G.A. Lawrence Hagarene iii : [...] unless this throw comes off crabs, there won’t be enough to bury me, unless I die a defaulter [F&H].
1890		Sporting Times 11 Jan. 1: [An] owner guilty of saying, when a dead certainty goes crabs, ‘Well, I’m bothered!’.
1938		(ref. to 18C–19C) H. Asbury Sucker’s Progress 42: A throw of two aces at Hazard has always been called a crab.

2. (US Und.) in the context of a robbery, loot that proves valueless, and thus irritatingly so.

1950

Goldin et al. DAUL 51/1: Crabs. [...] 2. Anything so valueless as to be irritating; nothing; pretzels. ‘The heavy (safe-cracking) ain’t no good no more. It’s a ball-breaker (discouraging thing) knocking off (robbing) keisters (safes) and getting crabs for your end (share).’.