This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 薽 is uncertain. It visually combines the grass radical 艹 with 甄, but the original semantic and phonetic roles are unclear.
The grass top 艹 suggests a plant, and the lower part 甄 is a rare shape. Picture an obscure weed with a tangled root system that is hard to identify.
For シン, imagine a shin-high weed in the grass: shin -> シン, and the plant is just a nameless thing at your shin.