The Collaborative International Dictionary of English v.0.48:

Blockhead \Block"head`\, n. [Block + head.]
   A stupid fellow; a dolt; a person deficient in understanding.
   [1913 Webster]

         The bookful blockhead, ignorantly read,
         With loads of learned lumber in his head. --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blockhead
    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]