This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
珒 combines 王 (jade) with 聿, but the exact historical development and original meaning are uncertain.
The left side 王 suggests jade, and the right side 聿 looks like a hand holding a brush. Imagine a jade ornament being crafted, but its exact meaning remains unclear.
For シン, picture a shiny jade ornament: shiny -> シン, and the jade piece is being polished.