Green’s Dictionary of Slang

1. (US) a small-time gambler [? one who keeps their ‘claws’ on their cash].

1873

J. O’Connor Wanderings of a Vagabond 361: By this time it appeared a heavier class of players had entered the place, and to make room for them at the table, the ‘crabbers’ were forced to vacate.

2. a fault-finder, a nag [crab v. (1)].

1918		N.Y. Tribune 19 May 8/5: A ‘dynamiter’ in engineer parlance, is the nth degree of knocker, crabber, kicker, sourball.
1939		T.J. Farr ‘The Language of the Tennessee Mountain Regions’ in AS XIV:2 90: crabber. A grouch.
1936		‘Josphine Tey’ Shilling for Candles 82: There were one or two crabbers, of course.