This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
祩 combines 示 (altar, spirit) with 朱 (vermillion), but the exact historical development and original meaning are uncertain.
The left side 示 suggests a religious or spiritual context, while the right side 朱 (vermillion) might hint at a ritual or curse. Without a clear meaning, picture a red-marked altar used in an obscure ceremony.
For シュ, imagine a shaman (シャーマン) performing a ritual at the altar: shaman -> シュ, tying the sound to the spiritual left component.