WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

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