n. 脚背, 背部, 脚背形的东西

[医] 足背, 足弓

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

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.