The Collaborative International Dictionary of English v.0.48:

Numskull \Num"skull`\, n. [Numb + skull.]
   A dunce; a dolt; a stupid fellow. [Also spelled numbskull.]
   [Colloq.]
   [1913 Webster]

         They have talked like numskulls.         --Arbuthnot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

numskull
    n 1: a stupid person; these words are used to express a low
         opinion of someone's intelligence [syn: dunce,
         dunderhead, numskull, blockhead, bonehead,
         lunkhead, hammerhead, knucklehead, loggerhead,
         muttonhead, shithead, dumbass, fuckhead]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "numskull":
   addlebrain, addlehead, addlepate, beefhead, blockhead, blubberhead,
   blunderhead, bonehead, bufflehead, cabbagehead, chowderhead,
   chucklehead, clodhead, clodpate, clodpoll, dolthead, dullhead,
   dumbhead, dunderhead, dunderpate, fathead, jolterhead, jughead,
   knucklehead, lunkhead, meathead, muddlehead, mushhead, muttonhead,
   noodlehead, peabrain, pinbrain, pinhead, puddinghead, pumpkin head,
   puzzlehead, stupidhead, thickhead, thickskull, tottyhead