The Collaborative International Dictionary of English v.0.48:

Thick-headed \Thick"-head`ed\, a.
   Having a thick skull; stupid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thickheaded
    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:

30 Moby Thesaurus words for "thickheaded":
   beetleheaded, blockheaded, blunt, blunt-witted, chuckleheaded, dim,
   dim-witted, dopey, dull, dull of mind, dull-headed, dull-pated,
   dull-witted, fat-witted, fatheaded, gross-headed, heavy,
   hebetudinous, numskulled, obtuse, slow, slow-witted, sluggish,
   thick, thick-brained, thick-headed, thick-pated, thick-witted,
   thickskulled, wooden