Daft means silly in a harmless way. A daft idea, don't be daft, daft as a brush.
It is British and affectionate rather than critical: don't be daft is usually reassurance. Daft as a brush is the standard comparison and nobody knows why a brush. In Old English the word meant gentle and mild-mannered, and the sense drifted from there.
1adjectivedaftSilly in a harmless way.
“What a daft idea, wearing sandals in the snow!”
daft is a quality.
Old English daefte meant gentle or meek. The sense slid from mild to simple to silly.
Proto-West Germanic
*daftī — fitting, suitable
Old English
dæfte — accommodating; gentle, meek, mild
Middle English
dafte
English, today
daft
Built from
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 (“good”). a 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).
Etymology from Wiktionary, CC BY-SA