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]