Meaning:
The phrase keep your shirt on means to calm down. This expression might be used in situations where a person is riled up or excited about something—it’s basically a way of saying relax.

The Origin Of ‘Keep Your Shirt On’
The origin of this phrase is unclear, however, it might have something to do with people getting angry. What I mean is, sometimes people get so angry that they get into physical fights with others. And if that happens, they might take off their shirt before the conflict starts. Why?
Well, shirts can restrict a person’s movement to an extent, especially if it’s a tighter fitting one. By removing the shirt, it allows the upper body to move around a bit more freely. It’s like wearing a pair of jeans, the legs can still move in them, but not as well.
Anyways, since angry people sometimes react by taking off their shirt for a fight, maybe the people not interested in having a brawl would tell them literally, to keep it on!
Now let’s talk about how old the idiom keep your shirt on is. The earliest I’ve seen it in print with its figurative meaning is from the late 19th century. I have two examples to show, the first is from the Geelong Advertiser newspaper, printed on June, 1894:
“Now young man, you keep your shirt on, and don’t be too smart.”
The second example is from the Melbourne Punch newspaper, June 1892, that reads:
“‘Keep your shirt collar on, Gladdy,’ says I.”
Example Sentence
- After an elderly woman cut in line, several people became annoyed with her, so I told them to keep their shirts on.
Related Example:
- I know you are excited for the miniature golf course, but cool your jets will you? I’ll be ready soon.