WordNet sense 1 (a person who is not very bright):

dolt, dullard, pillock, pudden-head, pudding head, poor fish, stupe, stupid person

WordNet sense 2 (having a mental age of between eight and twelve years):

moronic

WordNet sense 3 (having a mental age of three to seven years):

idiotic, imbecile

WordNet sense 4 (not using intelligence):

brainless, headless

WordNet sense 5 (lacking intelligence):

unintelligent

WordNet sense 6 (slow to learn or understand; lacking intellectual acuity):

dense, dim, dull, dumb, obtuse, slow

WordNet sense 7 (heavy and dull and stupid):

cloddish, doltish

WordNet sense 8 (used informally):

fatheaded, thick, wooden-headed

WordNet sense 9 (having or revealing stupidity):

foolish, anserine

WordNet sense 10 (in a state of mental numbness especially as resulting from shock):

dazed, stupefied, stunned

WordNet sense 11 (mentally confused; unable to think with clarity or act intelligently):

confused

WordNet sense 12 ((of especially persons) lacking sense or understanding or judgment):

senseless, witless