This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
晙 combines 日 (sun) with 夋, which likely contributes the sound. The exact historical development is uncertain.
The sun 日 beside the component 夋 suggests brightness, but the exact meaning is unclear. Picture a rare, bright day whose name you can't quite recall.
For シュン, imagine the sun shining soon after dawn: soon -> シュン, and the rare bright kanji appears.