thorn
莿 centers on a sharp, pointed plant growth: a thorn. It is a specific botanical term for a prickly projection.
莿 combines 艹 (grass/plant) with 刺 (thorn), reinforcing the idea of a thorny plant. The exact historical development is uncertain, but it likely originated as a specialized character for a thorny plant.
The grass top 艹 sits above 刺, which itself looks like a thorny plant. Picture a low-growing plant covered in sharp thorns: 莿 is a thorn.
For シ, imagine a single sharp thorn pricking your finger with a soft 'shi' sound: shi -> シ, and the thorn draws a tiny drop of blood.