This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
嚫 combines 口 (mouth) with 親, which likely contributes the sound. The exact historical development is uncertain.
The mouth radical 口 beside 親 suggests something spoken or related to a parent, but the meaning is unclear.
For シン, imagine a parent (親) speaking (口) a sincere (シン) word: シン comes from the parent's mouth.