This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
秼 combines 禾 (grain) with 朱, which may have contributed a phonetic element, but the exact historical development is uncertain.
The left side is grain 禾 and the right side is 朱 (vermillion). Without a clear meaning, picture a rare ritual where grain is dyed vermillion for a forgotten purpose.
For シュ, imagine a shoe filled with vermillion-dyed grain: shoe -> シュ, and the strange image helps recall the reading.