skin cracks or roughness
皸 centers on skin damage from dryness or cold: cracking, chapping, or roughness. It describes the visible splitting and coarseness of skin.
皸 combines 軍 (army, war) and 皮 (skin). The exact historical development is uncertain, but it likely relates to skin damage, perhaps from harsh conditions.
An army (軍) on the march in cold weather ends up with cracked skin (皮). Picture soldiers with rough, chapped hands.
For クン, imagine a soldier named Kun whose skin cracks in the cold: Kun -> クン, and his skin (皮) is rough from army (軍) life.
to chap; to crack