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.