名詞 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.（名詞）
The arched middle part of the human foot between the toes and the ankle.
腳背：人腳足尖和踝骨之間突起的中間部位
The part of a shoe or stocking covering the instep.
鞋或襪子的腳面部分

來源：
Middle English
中古英語