The Collaborative International Dictionary of English v.0.48:

boneheaded \bone"head*ed\ adj.
   very stupid; -- used of people or actions. [informal] --
   bone"head*ed*ness, n.

   Syn: blockheaded, fatheaded, loggerheaded, thick,
        thickheaded, thick-skulled, wooden-headed.
        [WordNet 1.5] bone-idle

WordNet (r) 3.0 (2006):

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