This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瑧 combines 王 (jade) with 秦, which likely contributes the sound. The exact historical development is uncertain.
The left side 王 suggests jade or a precious stone, and the right side 秦 is a phonetic component. Picture a rare, unnamed gemstone with the sound シン.
For シン, imagine a thin shin of jade: shin -> シン, a delicate sliver of the precious stone.