Source : Webster's Revised Unabridged Dictionary (1913)

Thick-skulled \Thick"-skulled`\, a.
   Having a thick skull; hence, dull; heavy; stupid; slow to
   learn.

Source : WordNet®

thick-skulled
     adj : used informally [syn: {blockheaded}, {boneheaded}, {fatheaded},
            {loggerheaded}, {thick}, {thickheaded}, {wooden-headed}]