sedge
莎 refers to a type of grass-like plant: sedge. It is a specific botanical term without broader abstract extensions.
莎 combines the grass radical 艹 with 沙, which likely provides the sound. The character was created to name a specific grass-like plant.
The grass radical 艹 sits above 沙 (sand). Picture a tough sedge plant growing out of sandy soil: 莎 is that sedge.
For サ, imagine the sedge growing in the Sahara: Sahara -> サ, and the sandy 沙 component reinforces the desert scene.
Carex scabrifolia (species of sedge)