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).
1550s, "to dip a little and often," hence "to wet by splashing," probably a frequentative of dab. Figurative sense of "do superficially" attested by 1620s. Related: Dabbled; dabbling. An Ellen Dablewife is in the Lancashire Inquests from 1336.
"apt or dexterous, subtly clever or skillful," mid-15c., from Old English gedæfte, which meant "mild, gentle, simple, meek," but which splintered into different forms and senses in Middle English, yielding this word and also daft (q.v.). In Middle English it also could mean "well-mannered, gentle, modest, mild," and "dull, uncouth, boorish." Cognate with Gothic gadaban "to be fit," Old Norse dafna "to grow strong," Dutch deftig "important, relevant," from Proto-Germanic *dab-, which has no certain IE etymology and is perhaps a substratum word. Related: Deftness.
c. 1200, "mild, well-mannered," Old English gedæfte "gentle, becoming," from Proto-Germanic *gadaftjaz (source also of Old English daeftan "to put in order, arrange," gedafen "suitable;" Gothic gadaban "to be fit"), from *dab-, which has no certain IE etymology and is perhaps a substratum word.
Sense deteriorated to "dull, awkward, uncouth, boorish" (c. 1300), perhaps via the notion of "humble." Further evolution to "foolish, simple, stupid" (mid-15c.) and "crazy" (1530s) probably was influenced by analogy with daffe "halfwit, fool, idiot" (see daffy); the whole group probably has a common origin. For sense evolution, compare nice, silly. Related: Daftly; daftness.