ill-judged, rash, foolish, foolhardy, reckless, madcap, wild, silly, stupid, ridiculous, preposterous, absurd, idiotic, asinine, imprudent, impracticable, unworkable, unrealistic, unconsidered, half-baked, ill-thought-out, ill-advised, ill-conceived
informal daft, crackpot, crackbrained, cock-eyed, crazy, barmy
foolish, silly, idiotic, unintelligent, empty-headed, scatterbrained, feather-brained, birdbrained, pea-brained, brainless, giddy
informal dippy, dizzy, dopey, dotty, airheaded
British informal dappy