The Collaborative International Dictionary of English v.0.48:

Half-wit \Half"-wit`\ (-w[i^]t`), n.
   A foolish person; a dolt; a blockhead; a dunce. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

half-wit
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]
    2: a stupid incompetent person [syn: dimwit, nitwit, half-
       wit, doofus]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "halfwit":
   ass, birdbrain, dolt, dullard, dunce, dunderhead, dunderpate, fool,
   idiot, imbecile, moron, nincompoop, ninny, nit, nitwit, numskull,
   simpleton, twit