The Collaborative International Dictionary of English v.0.48:

Blockheaded \Block"head`ed\, a.
   Stupid; dull.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blockheaded
    adj 1: (used informally) stupid [syn: blockheaded,
           boneheaded, duncical, duncish, fatheaded,
           loggerheaded, thick, thickheaded, thick-skulled,
           wooden-headed]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "blockheaded":
   beetleheaded, blockish, blunderheaded, boneheaded, cabbageheaded,
   chowderheaded, chuckleheaded, clodpated, dumbheaded, dunderheaded,
   fatheaded, jolterheaded, joltheaded, knuckleheaded, lunkheaded,
   muttonheaded, nitwitted, numskulled, pumpkin-headed, sapheaded,
   stupid, stupidheaded, thick, thickheaded, woodenheaded