The Collaborative International Dictionary of English v.0.48:

Thornback \Thorn"back`\, n.
   1. (Zool.) A European skate (Raia clavata) having thornlike
      spines on its back.
      [1913 Webster]

   2. (Zool.) The large European spider crab or king crab (Maia
      squinado).
      [1913 Webster]