wrinkles, cracking, creases
皴 centers on surface texture that is no longer smooth: wrinkles, creases, and fine cracking patterns, especially on skin or similar surfaces.
皴 combines 皮 (skin) with 夋, which likely contributes the sound and may relate to the idea of gathering or puckering. The exact historical development of the left component is uncertain.
The right side 皮 means skin, and the left side 夋 looks like a person with a furrowed brow. Picture skin that has gathered into wrinkles and creases, like a worried face.
For シュン, imagine a wrinkled face that suddenly smooths out with a quick 'shun' sound, like a magic spell: shun -> シュン, and the creases disappear.