fish guts
鯝 centers on the internal organs of a fish, specifically the guts or entrails.
鯝 combines 魚 (fish) with 固, which likely contributes the sound and may suggest the solid or fixed nature of the internal organs.
The left side is a fish 魚, and the right side 固 suggests something solid or fixed inside. Picture the solid, fixed guts inside a fish: 鯝 is fish guts.
For コ, imagine a fish's guts being examined by a curious child who says 'co' as they poke the solid insides: co -> コ.