This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
葴 combines 艹 (grass/plant) with 咸, but the exact historical development and original meaning are uncertain.
The top 艹 suggests a plant, and the bottom 咸 looks like a container. Imagine a mysterious plant growing out of a pot, its purpose unknown.
For シン, picture a shin-high plant in a pot: shin -> シン, and the plant is the unknown 葴.