Green’s Dictionary of Slang

dab n.1

[gaming jargon dab, a top-flight gamester; ? orig. schoolboy sl., the likely ety. is rooted in SE adept or dapper, but there is no positive proof of either; ‘Sl. Terms & the Gypsy Tongue’ in Baily’s Mag. Nov. 1871 suggests link to Hindi/Rom. dhab, dexterity]

[17C+] a skilful person, an expert.