This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 寯 is uncertain. It combines 宀 (roof) with 雋, but the original role of 雋 in this character is unclear.
The roof 宀 covers a complex lower shape 雋, but no clear meaning emerges from the parts alone. Treat the meaning as uncertain.
For シュン, imagine a rare, soon-to-be-forgotten character under a roof: soon -> シュン, and the character remains obscure.