bite, gnaw
嚙 centers on using teeth to cut or wear down: biting and gnawing.
嚙 combines 口 (mouth) with 齒 (teeth), directly expressing biting or gnawing with the mouth and teeth. It is the older traditional form; the modern simplified form is 噛.
A mouth 口 full of teeth 齒 is ready to bite and gnaw.
For コウ, imagine a cow biting and gnawing on grass: cow -> コウ, and the cow's mouth and teeth work together.