Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General Explanation of Thick Skin Talent

I would understand it as meaning:

base armor += 5 + man-at-arms*2

So, if man-at-arms is 0, the talent gives you +5 armor.
If man-at-arms is 1, the talent gives you +7 armor.
If man-at-arms is 2, the talent gives you +9 armor.

Haven't checked, but that's my understanding from your post.