close up, stop up
堙 centers on blocking or sealing an opening: closing something up or stopping it up so nothing can pass through.
堙 combines 土 (earth) with 垔, which likely contributes the sound and the idea of blocking or filling. The character suggests using earth to stop up or close an opening.
The left side is earth 土, and the right side 垔 looks like a container being filled. Picture piling earth into a hole to close it up completely.
For イン, imagine an inn with its entrance stopped up by a mound of earth: inn -> イン, and the blocked doorway keeps everything closed.