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1. To stay calm. In this usage, a possessive pronoun is often used between "keep" and "cool." You know they're going to try to antagonize you during the debate, so just keep cool. I always try to keep my cool when dealing with the cable company.
2. Literally, to not get overly hot, as in hot weather. A: "How have you been during this brutal heat wave?" B: "Oh, keeping cool, thanks to the air conditioning in the office."
Inf. to stay calm and undisturbed. Relax, man, keep cool! If Sally could just keep cool before a race, she could probably win.
keep one's cool
Inf. to remain calm and in control. Relax, man! Just keep your cool. It's hard to keep your cool when you've been cheated.
Also, keep a cool head; stay cool; be cool; (take it cool). Remain calm and under control, as in Keep cool, they'll soon show up, or Be cool, the surprise is not spoiled, or You have to keep a cool head in these volatile situations, or Sit tight, take it cool, they won't bother you again. All these terms employ cool in the sense of "not heated by strong emotion," a usage dating from the late 1300s or even earlier. The first three expressions are colloquial and date from the second half of the 1800s; both of the last two are slang, and the very last (take it cool) is the oldest, first recorded in 1841. Also see keep one's cool; play it cool.
keep your cool
COMMON If you keep your cool, you control your temper and stay calm in a difficult situation. Hilde's one of those born managers — keeps her cool in a crisis. I knew I had to keep my cool, but it was hard.
in. to keep calm. Now, keep cool. It’s going to be all right.