The Collaborative International Dictionary of English v.0.48:

fatheaded \fat"head`ed\ adj.
   same as stupid. Opposite of smart. [informal]

   Syn: blockheaded, boneheaded, loggerheaded, thick,
        thickheaded, thick-skulled, wooden-headed.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

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