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.