Meaning

adjective informal or slang terms for mentally irregular

originFrom Middle English dafte, defte (“gentle; having good manners; humble, modest; awkward; dull; boorish”), from Old English dæfte (“accommodating; gentle, meek, mild”), from Proto-West Germanic *daftī (“fitting, suitable”). Related to Old English dafnian, dafenian (“to be fitting, appropriate, or becoming”), Russian до́брый (dóbryj, “good”). Doublet of deft. Compare silly which originally meant…

Etymology from Wiktionary, CC BY-SA 3.0