This is a word I rather enjoy using because it often fits so many people.

daft (daft): / daft /
adjective

(1) senseless, irrational, stupid, or foolish; lacking common sense
(2) insane, mad, or crazy
(3) playful, frolicsome, or merry (Scottish)

[From Middle English dafte meaning “uncouth, awkward, stupid; meek, gentle, mild,” from Old English dæfte from Old English gedæfte meaning “fitting or meek.”]

Usage: Don’t be daft!