sardine
鰛 centers on a specific fish: the sardine. The meaning is narrow and concrete, referring to this small, silvery fish.
鰛 combines 魚 (fish) with 昷, which likely contributes the sound オン. The character was created to represent the sardine specifically.
The left side is a fish 魚, and the right side 昷 looks like a container with a lid. Imagine a can of sardines packed tightly together.
For オン, picture turning on a can opener to open a tin of sardines: on -> オン, and the fish is ready to eat.