Here's the breakdown:
* Historical Context: During the Civil War, soldiers often wore their shirts tucked into their trousers. If a soldier was getting ready for a fight, he would often unbutton his shirt to allow for greater freedom of movement. This was seen as a sign of readiness for battle.
* Meaning: The idiom "keep your shirt on" means to calm down or be patient, often used in situations where someone is becoming agitated or impatient. It implies that there's no need to rush into action or get worked up.
* Evolution of Meaning: The idiom likely developed because, by keeping their shirt on, soldiers were signaling that they were not immediately preparing for battle, which would indicate a more peaceful approach. This became associated with a calmer demeanor, and the phrase evolved to mean to be patient or remain calm.
In essence, "keep your shirt on" is a way of telling someone to relax and not get overly excited or aggressive, just like a soldier would keep his shirt on to indicate that he wasn't going into battle.