English Idiom - Keep Your Shirt On! The idiom "keep your shirt on" is used to tell someone to remain calm or patient in a situation. It's a colloquial expression that originated in the United States in the early 20th century. The exact origin is uncertain, but it likely stems from the idea that when people become agitated or angry, they might start to remove their clothing in preparation for a physical altercation. So, telling someone to "keep your shirt on" is a way of urging them to maintain t