mat, coverlet, underwear
裀 centers on a cloth item placed under the body: a mat, a coverlet, or underwear. The common thread is a layer of fabric that provides a barrier or cushion between the body and another surface.
裀 combines 衤 (clothing) with 因, which likely contributes the sound and may suggest something that is placed underneath or relied upon, fitting the idea of a mat or undergarment.
The clothing radical 衤 beside 因 suggests a cloth item you rely on: picture a soft mat or coverlet you place under yourself, or underwear as the layer closest to the skin.
For イン, imagine an inn where every bed has a fresh mat and clean underwear: inn -> イン, and the clothing radical 衤 reminds you it's fabric.