This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瞚 combines 目 (eye) with 寅, which likely contributes the sound. The exact historical development is uncertain.
The left side is an eye 目, and the right side 寅 looks like a complex shape. Without a clear meaning, picture an eye peering through an ornate screen, but the image is blurry.
For シュン, imagine the eye blinking quickly with a soft shun sound: shun -> シュン, as the eyelid shuts and opens.