This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 夋 is uncertain. It visually combines 允 and 夂, but the original component relationship is unclear.
The top 允 suggests a person, and the bottom 夂 looks like legs. Picture someone crouching down, which fits the kun reading うずくま.る (to crouch).
For シュン, imagine a person crouching soon after a long run: soon -> シュン, and they crouch to rest.