keep one's shirt on
keep one's shirt on {v. phr.}, {slang}
To calm down; keep from losing your temper or getting impatient or excited.
Bob got very angry when John accidentally bumped into him, but John told him to keep his shirt on.
- Usually used as a command; may be considered impolite.
John said to Bob, "Keep your shirt on."
Contrast: GET ONE'S DANDER UP.
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Source: A Dictionary of American Idioms