middle
弣 refers to the middle part of a bow, the grip where the archer holds it. It is a specialized term for that central section.
弣 combines 弓 (bow) with 付, which likely contributes the sound and may relate to attaching or the central part. The exact historical development is uncertain.
The left side is a bow 弓, and the right side 付 looks like a person attaching their hand to the middle of the bow. Picture gripping the center of a bow where it is held.
For フ, imagine the sound of a bowstring being pulled back and released: 'fu!' The middle of the bow vibrates with a 'fu' sound.