Source : Webster's Revised Unabridged Dictionary (1913)

Blockhead \Block"head`\, n. [Block + head.]
   A stupid fellow; a dolt; a person deficient in understanding.

         The bookful blockhead, ignorantly read, With loads of
         learned lumber in his head.              --Pope.

Source : WordNet®

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