keep one's shirt on

keep (one's) shirt on

To remain calm and wait for something. Typically used as an imperative in the phrase "keep your shirt on." Keep your shirt on, dude, the pizza will be here soon enough. It would have gone smoothly if Larry had been able to keep his shirt on instead of trying to get in early.
See also: keep, on, shirt

keep one's shirt on

Stay calm, be patient; not give way to temper or excitement. For example, Keep your shirt on, Bob, they'll be here in time for the wedding. [Colloquial; mid-1800s]
See also: keep, on, shirt