This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
齓 combines 齒 (tooth) with 乚, but the historical development and original meaning are uncertain.
The tooth radical 齒 suggests a dental connection, but the right side 乚 is obscure. Picture a hidden tooth tucked away in a curved corner.
For シン, imagine a shin (シン) bone being gnawed by a toothy creature, though the meaning is uncertain.