This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 茋 is uncertain. It visually contains 艹 (grass) and 氐, but the original semantic and phonetic roles are unclear.
With 艹 (grass) on top and 氐 below, picture a plant whose meaning is obscure, like a mysterious herb growing in a hidden place.
For シ, imagine a single leaf (シ as in し, one) of this mysterious plant: シ -> し, and the grass 艹 reminds you it's a plant.