wait for, wait until, as soon as
竢 centers on the idea of waiting for a specific moment or condition: to wait until something happens, or to act as soon as it does.
竢 combines 立 (stand) with 矣, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with waiting or standing by.
A person standing (立) beside an arrow-like shape (矣) that has just been shot, waiting for it to land: 竢 means waiting for the right moment.
For シ, imagine standing and waiting for a sheep (シ sounds like 'she') to cross the road: sheep -> シ, and you wait until it passes.