"Keep your shirt on"/"Keep your pants on."

Which of those two expressions came first? I was thinking "pants" was first and then it was cleaned up to "shirt." Chris states confidently that "shirt" was first--the point was that a man would take off his shirt when he was preparing to fight. Wouldn't it have been "Keep your jacket on" first, then? And how did "pants" ever get started?

The internet says:

Before modern manufacturing techniques, shirts, and all clothes for that matter, required a lot of labor to make. They were more expensive than they are today. Someone thinking of starting a fight might take off his shirt to prevent damage. Telling someone to "keep his shirt on" was equivalent to telling him "I don't want to fight." ...

Hmmm... so then, it got dirtied up.