nock at end of bow, stretch
彄 centers on the notch at the end of a bow where the string is attached, and by extension the act of stretching the bowstring to fit it. The meanings are tightly linked to archery equipment and action.
彄 combines 弓 (bow) with 區, which likely contributes the sound and may relate to the idea of a receptacle or compartment, fitting the notch that holds the bowstring. The exact historical development is uncertain.
The left side is a bow 弓. The right side 區 looks like a container with a small opening, like the notch at the end of a bow that holds the string. Picture stretching the bowstring to hook it into that notch.
For コウ, imagine the bowstring snapping into the notch with a loud 'ko' sound, and then you stretch it further: 'ko' -> コウ.