instep

名词 instep:

1. the arch of the foot
2. the part of a shoe or stocking that covers the arch of the foot

in.step

n.（名词）
The arched middle part of the human foot between the toes and the ankle.
脚背：人脚足尖和踝骨之间突起的中间部位
The part of a shoe or stocking covering the instep.
鞋或袜子的脚面部分

来源：
Middle English
中古英语