Fashion has not only determined people’s dress, but their way of speaking. During the last century, men’s shirts had stiffly starched fronts. Though looking elegant, they constricted the wearer’s movements. So, attired, it would be very difficult and disadvantageous for him to engage in fisticuffs, not uncommon at the time. To do so successfully, he had to remove his shirt first.
When a heated argument threatened to get out of hand and develop into a physical fight, an opponent anxious to avoid it, called on his adversary to ‘keep his shirt on’. Thus, to calm down someone in a belligerent mood, you yelled “keep your shirt on.”