This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蜄 combines 虫 (insect) with 辰, which likely contributes the sound. The exact historical development is uncertain.
The insect radical 虫 suggests a bug or creature, but the overall meaning is unclear. Imagine a mysterious insect that moves in an unusual way.
For シン, picture a shin (shin) bone of a tiny insect twitching as it moves: shin -> シン.