Clever, skilled, as “a dab-hand at it”; a corrupt contraction of the Latin adeptus (an adept). “Dabster” is another form. Apt is a related word.
An Eton stripling, training for the law. A dunce at learning, but a dab at taw (marbles).
Anon: Logic; or, The Biter Bit
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894