the instep
跗 refers specifically to the instep, the arched upper surface of the foot between the toes and the ankle.
跗 combines 足 (foot) with 付, which likely contributes the sound and may relate to the idea of attaching or being placed upon, referring to the instep as the upper part of the foot.
The left side 足 means foot, and the right side 付 suggests attaching. Picture the instep as the part of the foot where a shoe strap attaches across the top.
For フ, imagine a shoe that is too tight on the instep, making a 'foo' sound as you try to squeeze your foot in: foo -> フ.