I, myself, katakana mu radical (no. 28)
厶 centers on the self: the first-person concept of 'I' or 'myself.' The radical sense is the same visible shape used as a component in other characters.
厶 is a pictograph of a bent arm or a self-referential shape, associated with the idea of 'self' or 'private.' The exact historical development is uncertain.
The shape 厶 looks like a person pointing to themselves with a bent arm, saying 'I' or 'myself.'
For シ, imagine the shape as a person pointing to themselves and saying 'she' — but it's really 'I': she -> シ.