cushion, mattress
茵 centers on a soft padded surface for sitting or lying: a cushion or mattress.
茵 combines 艹 (grass) with 因, which likely contributes the sound and may relate to something spread out or underlying. The character suggests a grass mat or cushion.
The grass radical 艹 sits on top of 因, like a soft grassy cushion spread over a base. Picture a thick mat of woven grass used as a mattress.
For イン, imagine sinking into a soft inn mattress: inn -> イン, and the cushion feels like a cozy inn bed.