servant
廝 centers on a person who serves or does menial work: a servant or attendant.
廝 combines 广 (building) with 斯, which likely contributes the sound and may relate to splitting or menial work. The exact historical development is uncertain.
Under a roof 广, the complex shape 斯 looks like a busy servant moving between tasks. Picture a servant in a large house, always working.
For シ, imagine a servant saying 'she' when referring to their master: she -> シ, and the servant serves her.