wait, depend on
俟 centers on waiting and reliance: pausing for something or someone, and depending on them to act or arrive.
俟 combines 亻 (person) with 矣, which likely contributes the sound and may relate to a pause or completion, suggesting a person waiting.
A person 亻 beside 矣, which looks like an arrow 矢 with a private mark 厶, as if someone is waiting for a signal to act.
For シ, imagine a person waiting to see (シ) if the arrow will be released: see -> シ, and the person depends on the signal.