fall
霣 centers on falling, especially of rain or something descending from the sky. The given meaning 'fall' is its core idea.
霣 combines 雨 (rain) with 員, which likely contributes the sound and may relate to falling or dropping. The exact historical development is uncertain.
Rain 雨 falls from the sky, and 員 looks like a person or object tumbling down. Together, 霣 means something falling, like rain or a descending object.
For イン, imagine rain falling on an inn: inn -> イン, and the rain keeps falling on the roof.