SYNONYMS
stupid, unintelligent, ignorant, brainless, mindless, foolish, slow, slow-witted, dull-witted, witless, doltish, blockish, dunce-like, simple-minded, empty-headed, vacuous, vapid, half-witted, idiotic, moronic, imbecilic, obtuse, bovine, lumpish
gullible, naive
informal thick, dim, dumb, dopey, dippy, dozy, cretinous, thickheaded, chuckleheaded, dunderheaded, wooden-headed, fat-headed, thick-skulled, muttonheaded, boneheaded, lamebrained, birdbrained, pea-brained, pig-ignorant, slow on the uptake, soft in the head, brain-dead, dead from the neck up
British informal daft, not the full shilling, as thick as two short planks
South African informal dof
West Indian informal dotish
North American vulgar slang dumb-ass