WordNet (r) 3.0 (2006):

dimwit
    n 1: a stupid incompetent person [syn: dimwit, nitwit,
         half-wit, doofus]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "dimwit":
   Boeotian, block, clod, cluck, dolt, donkey, dope, dullard, dumb,
   dumb cluck, dumbbell, dummy, dunce, gowk, jobbernowl, lackwit,
   lamebrain, lightweight, looby, loon, niais, nincompoop, ninny,
   ninnyhammer, nitwit, noddy, put, stupid, thickwit, witling