Source : Webster's Revised Unabridged Dictionary (1913)

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

         They have talked like numskulls.         --Arbuthnot.

Source : WordNet®

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