early 14c., dabben "to strike," of unknown origin, perhaps imitative. Compare Old Norse dabba "to tap, slap." Modern sense of "strike gently with the hand, strike with a slight, quick pressure" developed by mid-16c., influenced by French dauber (see daub). Related: Dabbed; dabbing; dabber.
As a noun from c. 1300, "heavy blow with a weapon," later "gentle blow with the hand or some soft substance" (1755). Meaning "small lump or mass of something soft" is from 1749. Dab hand is British slang, 1828, from dab "expert, knowing or skillful person" (1690s), said by OED to be "school slang," of unknown origin, perhaps from dab in the "strike lightly" sense. Compare dabster, which meant both "an expert" (1708) or "a bungler" (1871, perhaps by confusion with daub).