one thousand millions, sometimes used for one hundred millions
秭 centers on an extremely large number: specifically one thousand millions, though it has also been used for one hundred millions. The core idea is a vast, almost uncountable quantity.
秭 combines 禾 (grain) with a phonetic component that likely contributed the reading. The exact historical development of the right side is uncertain, but the character was used to represent a specific large number.
Picture a vast field of grain (禾) so enormous that counting the stalks would reach one thousand millions. The right side looks like a tally mark trying to keep count but failing against the sheer quantity.
For シ, imagine a single grain (禾) representing 'she' (シ) — a queen of numbers — because she rules over one thousand millions.