Keep your shirt on
In every girl’s way of living there are moments when she forgets the formality of dressing up and returns to the well-seasoned, any season look of a shirt. Cooler than a sweater and without the interruption of frills. Elizabeth Dickson earmarks the shirt born to the manner of classical elegance.
advertisement from The Tatler (London) of 26th June 1963
Of American origin, the phrase keep your shirt on means don’t lose your temper, stay calm—cf. the synonymous keep your hair on.
It is based on the image of taking off one’s shirt before getting into a fight. This is illustrated by the following passage from Colonial Adventures and Experiences. By a University Man (London, 1871), by George Carrington:
“Come on,” he shouted, “come on, you wretch, and fight;” he was so excited that he did not stammer. “What for?” said I, much amazed. “I’ll let you know what for; you’ve killed my mate; come on.” All this time he was making attitudes, and trying to get his shirt off, preparatory to pitching into me. “Don’t be an ass, P.,” I said, walking up to him, “keep your shirt on; you’ll catch your death of cold. C. is only asleep; he’ll be all right in the morning.”
The earliest instance of the phrase that I have found is from The Ohio Democrat (New Philadelphia, Ohio) of 18th April 1844:
KEEP YOUR SHIRTS ON FEDS¹
Some of the coons² have been chuckling for a week past with the idea that the Democrat was about to blow out, as Pat predicted more than 2 years ago. We are sorry to spoil their fun, but we must inform them that we have received a fresh supply of paper, and are good for 3 months longer any how.
¹ Fed: a member or supporter of the Federalist Party (also known as the Federal Party), which was opposed by the original Democratic Party (also known as the Anti-Federal Party, the Republican Party and the Democratic Republican Party). In the 1820s, the Democratic Party split into factions, one of which became the modern Democratic Party, in opposition first to the Whig Party then, from the 1850s, to the modern Republican Party.
² coon: a nickname for a member of the Whig Party, which at one time had the racoon as an emblem.
Another early instance is from The Daily Delta (New Orleans, Louisiana) of 8th December 1849:
Bayou Sara³ Correspondence of the Delta.
Bayou Sara, La., Dec. 2, 1849.
Dear Delta: […].
You talk of the bustle and business of your beautiful Crescent City⁴, but, let me admonish thee, “keep your shirt on”—for, in ratio to its population, our good little village does business to an extent even greater than your modern Babel, or “Tyre⁵,” as in your vanity you sometimes call it.
³ Bayou Sara: a port on the Mississippi River, between New Orleans and Memphis.
⁴ Crescent City: New Orleans; according to John Rushing, this nickname alludes to the fact that most of the area that is now New Orleans was originally an uninhabitable bog, the only viable land on which to build being at that time a crescent-shaped sliver along the bend in the Mississippi River where the old part of town is.
⁵ Tyre: a port on the Mediterranean in southern Lebanon; founded in the 2nd millennium BC as a colony of Sidon, it was for centuries a Phoenician port and trading centre.