This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
愖 combines 忄 (heart) with 甚, but the exact historical development is uncertain. It may have originally expressed a heart-related meaning associated with the sound of 甚, but the character is rare and its origin is not well documented.
The heart radical 忄 beside 甚 suggests a state of mind, but the exact meaning is unclear. Imagine a heart feeling something intense or extreme, as 甚 can mean 'extreme'.
For シン, imagine a person named Shin whose heart is extremely moved by something: Shin -> シン, and the heart 忄 feels it deeply.