This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 囙 is uncertain. It appears to be a variant or ancient form related to 因, but the component relationship is not clearly established.
The outer enclosure 囗 suggests something contained, but the inner shape is unclear. Without a known meaning, treat it as a mysterious box.
For イン, imagine an inn with a strange sign shaped like 囙: inn -> イン, and the unusual kanji marks the entrance.