thorn
朿 centers on a sharp, pointed protrusion: a thorn. It is a pictographic character that visually represents a thorny plant or spike.
朿 is a pictograph of a thorn or spike, with the vertical stroke representing the thorn and the surrounding strokes suggesting a plant or branch. The modern form retains this pointed, spiky appearance.
The vertical line down the middle is the sharp thorn, and the surrounding strokes look like the branches or leaves of a thorny plant. Picture a single thorn sticking out from a twisted stem.
For シ, imagine a sharp thorn making a soft 'shhh' sound as it scratches something: shhh -> シ, like the hiss of a thorn dragging across a surface.