Origin of "Keep Your Shirt On"

This expression comes from England, around the year 1600 on.
The current murder rate for the UK a couple of years ago was 1.6 per 1000,000. But between 1600 and 1700 it was seven times that. Fighting was still the most common way to resolve differences, but fighters would remove their shirts before the fight. Clothes were still hand-made and too expensive to get torn and bloodstained. If during an argument someone took their shirt off, it only meant one thing. So telling someone to keep their shirt on became equivalent to telling them to calm down.