skin cracks or roughness
皹 centers on skin that has become rough or cracked, typically from dryness or cold. The meaning covers both the visible cracking and the resulting rough texture.
皹 combines 皮 (skin) with 軍, which likely contributes the sound. The exact historical development is uncertain, but the phono-semantic structure is typical for characters describing skin conditions.
The left side 皮 means skin, and the right side 軍 looks like a military vehicle with tracks that can crack the ground. Imagine skin so dry it cracks like earth under a tank.
For クン, picture a military unit (軍, ぐん) marching until their skin cracks from the harsh conditions: the ぐん sound shifts slightly to クン as the skin roughens.
chaps (in the skin); cracks