[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.