cut, divide, companion, follower
厮 centers on a person in a subordinate role: a companion or follower who does menial work, often involving cutting or dividing tasks.
厮 combines 厂 (cliff) with 斯, which likely contributes the sound and the idea of splitting or menial work. The exact historical development is uncertain.
Under a cliff 厂, a follower 斯 is cutting and dividing tasks. Picture a servant splitting firewood at the base of a cliff.
For シ, imagine the follower saying 'she' cuts the wood: she -> シ, and she is the companion doing the dividing.