Green’s Dictionary of Slang

(UK juv.) useless, inadequate.

1905		H. Green Actors’ Boarding House (1906) 241: The show is crab.
1993		Source May 52: I got crab rappers trying to put a brother down like I’m a punk.
2001		OnLine Dict. of Playground Sl. 🌐 crab adj. rubbish at doing a thing, e.g. ‘He’s crab at that!’.