to chop straw fine for animals
莝 centers on cutting straw into fine pieces for animal feed. The meaning is narrow and concrete, with no clear abstract extensions.
莝 combines the grass radical 艹 with 坐, which likely contributes the sound and relates to cutting or chopping. The exact historical development is uncertain.
The grass radical 艹 sits above 坐, like chopped straw piled up after being cut fine for animals.
For サ, imagine a saw cutting through straw: saw -> サ, and the blade chops the grass into fine pieces.