The Collaborative International Dictionary of English v.0.48:

Hardhead \Hard"head`\ (-h[e^]d`), n.
   1. Clash or collision of heads in contest. --Dryden.
      [1913 Webster]

   2. (Zool.)
      (a) The menhaden. See Menhaden. [Local, U. S.]
      (b) Block's gurnard (Trigla gurnardus) of Europe.
      (c) A California salmon; the steelhead.
      (d) The gray whale. See Gray whale, under Gray.
      (e) A coarse American commercial sponge (Spongia dura).
          [1913 Webster] hardheaded