This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
秄 combines 禾 (grain) and 子 (child). The exact historical development is uncertain, but it likely relates to cultivating grain or a phonetic loan.
The left side is grain 禾 and the right side is a child 子. Picture a child tending a small grain plant, helping it grow.
For シ, imagine a child 子 whispering 'shh' to the grain 禾 as it grows: shh -> シ.