shirt 衬衫，衬衣，字面来看，Keep your shirt on 穿上衬衫的意思；
In English, To keep calm. 在英文里，是保持冷静，别激动的意思； If we find ourselves in a heated situation with a person getting riled up, we may try calming the irritated person by using this expression.如果我们发现周围有人发怒或者生气了，我们就可以用keep your shirt on去让他保持冷静；
This phrase may originate from the fact that when people get angry and are looking to fight, they'll sometimes remove their shirt.
Why? Because removing one's shirt increases maneuverability；
it allows for further range of motion of the upper body that would otherwise be limited.
Therefore, those who were not interested in fighting would advise an angered person to literally keep their shirt on, showing they wanted to settle things through peaceful means rather than through violence.
keep your shirt on, we have plenty of time to catch the train.
Dad, keep your shirt on, please! I'll study hard next term.爸爸，千万不要发火，我下学期一定努力学习。