This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
惞 combines the heart radical 忄 with 欣, which likely contributes sound and the idea of joy or gladness. The exact historical development is uncertain.
The heart radical 忄 beside 欣 suggests a feeling of joy or gladness, but the exact meaning is unclear.
For キン, imagine a joyful king (キン) whose heart is full of gladness: king -> キン, and the heart radical 忄 shows the feeling.