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]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "blockhead":
   addlebrain, addlehead, addlepate, beefhead, blubberhead, blunderer,
   blunderhead, bonehead, boor, botcher, bufflehead, bumbler, bungler,
   cabbagehead, chowderhead, chucklehead, clod, clodhead, clodhopper,
   clodknocker, clodpate, clodpoll, clot, clown, dimwit, dolt,
   dolthead, dullhead, dumbbell, dumbhead, dummy, dunderhead,
   dunderpate, fathead, fumbler, gawk, gowk, jolterhead, jughead,
   klutz, knucklehead, looby, lout, lubber, lunkhead, meathead,
   muddlehead, mushhead, muttonhead, nitwit, noodlehead, numskull,
   oaf, ox, peabrain, pinbrain, pinhead, puddinghead, pumpkin head,
   puzzlehead, slouch, slubberer, stupidhead, thickhead, thickskull,
   tottyhead, yokel