Keep your shirt on is an old phrase used in a time when men would take off their shirts before they got in a fight. A classy way of saying chill out or don't over react
by gojanego1234321 September 12, 2009
it means that you have no need to act all polite or formal. It also means that you dont have to be such a stuck up person by trying to act cool all the time.
by Artemicion January 4, 2016