SYNONYMS
simple-minded, simple, stupid, idiotic, moronic, imbecilic, dull-witted, dull, dim-witted, slow-witted, slow, witless, half-witted, feeble-minded, dunce-like, cretinous, empty-headed, vacuous, vapid
deranged, unhinged, insane, mad
informal touched, thick, dim, dopey, dumb, dozy, birdbrained, pea-brained, pig-ignorant, bovine, slow on the uptake, soft in the head, brain-dead, boneheaded, lamebrained, chuckleheaded, dunderheaded, wooden-headed, fat-headed, muttonheaded, not all there, not quite right, crazy, mental, nuts, nutty, cracked, cuckoo, bonkers, dippy
British informal not the full shilling, dotty, potty, batty, crackers, barmy, thick as two short planks
North American informal dumb-ass