The Collaborative International Dictionary of English v.0.48:

Instep \In"step\, n. [Formerly also instop, instup.]
   [1913 Webster]
   1. The arched middle portion of the human foot next in front
      of the ankle joint.
      [1913 Webster]

   2. The part of a shoe, boot, or stocking covering the
      instep[1].
      [PJC]

   3. That part of the hind leg of the horse and allied animals,
      between the hock, or ham, and the pastern joint.
      [1913 Webster]