The Collaborative International Dictionary of English v.0.48:

Thickhead \Thick"head`\, n.
   1. A thick-headed or stupid person. [Colloq.]
      [1913 Webster]

   2. (Zool.) Any one of several species of Australian singing
      birds of the genus Pachycephala. The males of some of
      the species are bright-colored. Some of the species are
      popularly called thrushes.
      [1913 Webster]