daft

From 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 “blessed; good, innocent; pitiful; weak”, but now means “laughable or amusing through foolishness or a foolish appearance; mentally simple, foolish”.

Unrelated to, though perhaps influenced by, daff (“fool (n.); to be foolish (v.)”) (past form daffed).