privy, toilet
厠 centers on a small, private room for bodily functions: a toilet or privy.
厠 combines 厂 (cliff or enclosure) with 則, which likely provides the sound. The character originally referred to a small side room or enclosure, later specializing to mean toilet.
Under the cliff enclosure 厂, the inner component 則 looks like a rule or pattern. Imagine a small, private room tucked away under a cliff where one follows the 'rules' of hygiene: that's the toilet.
For シ, picture a shy person using the privy: 'She' (シ) goes in quietly. The sound 'she' cues シ while the scene fits the private toilet meaning.