cool as a cucumber Calm, cool, and collected; self-possessed, composed. Cucumbers have long been used in salads and relishes for their refreshing, cooling quality. This popular simile dates from 1732.

cool your jets Relax, calm down, take it easy; used chiefly as an admonition. This recent American slang expression is perhaps an extension of the 1950s slang term cool it. The jets in the phrase may refer to the jet engines of a plane which get extremely hot before takeoff, and are thus comparable to the feverishly excited condition of an individual to whom this remark would be addressed.

count to ten To take a deep breath, calm down, and gird one-self to do something difficult or trying; to pause and consider before acting impetuously; to redirect one’s energy and attention to avoid becoming enraged. This common expression is often used by someone who is violently angry and on the verge of losing his temper. It is a warning to another person to behave in a certain manner or suffer the consequences when the counter reaches “ten.”

hold your horses Hold on, be patient, keep calm, don’t get excited; nearly always used in the imperative. The allusion is to the way a driver holds his horses back by pulling up on the reins in order to slow them down. Of U.S. origin, this expression is thought to have first appeared in print in its figurative sense in the New Orleans Picayune (September, 1844):

keep one’s powder dry To keep cool, to keep control, to remain calm and ready for action. This expression is military in origin and refers to the reputed final words of Sir Oliver Cromwell to his troops before they crossed a river to attack on the opposite side:

keep your shirt on Stay calm, keep cool, don’t get worked-up; also hold on to your shirttail; both expressions nearly always used in the imperative. Men usually remove their shirts before engaging in a fistfight; whence the expression. George W. Harris used this U.S. slang phrase in the Spirit of the Times (N.Y., 1854):

on an even keel Steady, stable, balanced; even-tempered; maintaining composure or equilibrium. Keel is a nautical term for a “central fore-and-aft structural member in the bottom of a hull” (Random House Diet.) which affects a vessel’s stability. Nautical use of on an even keel, as in the following quotation from James Greenwood’s A Rudimentary Treatise on Navigation (1850), has given rise to current figurative use of this expression.

roll with the punches See ENDURANCE.

without turning a hair Without batting an eyelash, showing no sign of excitement or emotion; completely calm and composed, unperturbed, unflustered.

The earliest recorded literal use of the expression is found in Jane Austen’s Northanger Abbey (1798) in allusion to a horse which, though hot from racing, did not become sweaty or ruffle its hair.