This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
洇 combines 氵 (water) with 因, which likely contributes sound. The exact historical development is uncertain.
Water 氵 beside 因 suggests something sinking or soaking in, but the exact meaning is uncertain.
For イン, imagine water 氵 seeping into an inn イン: inn -> イン, and the water soaks in.