Green’s Dictionary of Slang

skilled, expert.

1831		Satirist (London) 24 Apr. 24/1: Many fine fish of the dab kind have been taken in [...] in the pool attached to the house [i.e. Crockfords ’casino]. The usual bait for such fish is gudgeon paste mixed up with pigeon's milk.
1887		‘’Arry at the Sea-Side’ in Punch 10 Sept. 111/1: If a feller would tackle a feminine fair up to Dick, he ’as got to be dabs at the cackle.
1914		A. Brazil Leader of the Lower School 181: ‘[S]he ought to be a dab critic of scenery by now’.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: dab. Expert, extremely proficient. An abbreviation of ‘Dabster’.