the insect, commonly known as the silver-fish, which destroys books, clothing, etc.
蟫 refers to a specific insect known for damaging books and clothing, commonly called the silverfish. Its meaning is narrow and concrete, centered on this destructive household pest.
蟫 combines the insect radical 虫 with the phonetic component 覃, which likely contributed the sound. The character was created to name a specific insect, the silverfish.
The insect radical 虫 on the left tells you it's a bug. The right side 覃 looks like a complex, old-fashioned shape, hinting at an insect that hides in old books and clothes. Picture a silverfish crawling out of an ancient, dusty tome.
For イン, imagine an inn with old, musty books where silverfish thrive: inn -> イン, and the insect 蟫 is found there.