shell
厴 centers on a hard protective outer covering: a shell, like that of a mollusk or turtle.
厴 is a phono-semantic compound with 甲 (shell, armor) inside and 厭 likely contributing the sound エン. The exact historical development is uncertain.
Inside the outer shape 厭, you see 甲, which looks like a turtle's shell with a cross pattern. Picture a hard shell enclosing and protecting what's inside.
For エン, imagine an enormous shell that echoes when you tap it: the 'en' sound reverberates inside the shell, cueing エン.
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operculum (of a shellfish)