1 TEMPERATURE low in temperature, but not cold, often in a way that feels pleasant: There was a cool breeze blowing off the sea. | sipping a cool drink-see cold 12 CALM calm and not nervous, upset, embarrassed etc: Now just stay cool. Everything's OK. | (as) cool as a cucumber (=very calm): Robert walked into the exam looking cool as a cucumber. | cool customer (=someone who behaves calmly in a difficult situation) | cool head (=ability to remain calm in a difficult situation): The job is quite demanding, so we need someone with a cool head. | cool, calm, and collected (=calm): Although she was nervous before the interview she managed to appear cool, calm and collected.3 NOT FRIENDLY behaving in a way that is not as friendly as you expect: Her gaze was decidedly cool.(+ towards): The boss didn't actually say anything critical, but he was very cool towards me.4 FASHIONABLE informal very attractive, fashionable, relaxed etc, in a way that people admire: You look really cool in those sunglasses.5 it's cool spoken used to say that something is not a problem: Don't worry about the work - it's cool!6 COLOUR a cool colour is one, such as blue or green, that makes you think of cool things7 a cool million/hundred thousand etc informal a surprisingly large amount of money that someone seems to earn very easily: He earns a cool half million every year.- coolness noun (U) - coolly adverb - coolish adjective 2 verb1 (I, T) also cool down to become cool or cooler, or make something do this: You'll need to let your tea cool before you drink it. | They opened the windows to cool the room down.2 (I) if a feeling, emotion, or relationship cools, it becomes less strong: Our initial enthusiasm cooled when we saw how much work was involved.3 cool it spokena) used to tell someone to stop being angry, violent etc: That's enough arguing, you two - cool it!b) to stop putting as much effort into something, or pressure on someone as you have been: The more you chase after him, the less likely he is to go out with you. You'd better cool it a bit.4 cool your heels to be forced to wait: The receptionist kept me cooling my heels for at least an hour.cool down phrasal verb1 (I, T) to become cool or cooler, or make something do this: Let the engine cool down, and then try starting it. | cool sth down: Blow on your cocoa to cool it down.2 (I) to become calm after being angry: His father took a long time to cool down after their last argument.cool off phrasal verb (I)1 to return to a normal temperature after being hot: We'd been in the sun all day, so went for a swim to cool off.2 to become calm after being angry: Maybe you should go away and cool off before we talk any more.3 noun1 the cool a temperature that is pleasantly cold(+ of): They went for a stroll in the cool of the evening.2 keep your cool to remain calm in a frightening or difficult situation3 lose your cool to stop being calm in an annoying or frightening situation: I couldn't help it, I just lost my cool and started shouting at him.4 adverb play it cool to behave in a calm way because you do not want someone to know that you are really nervous, angry etc: Don't worry, just listen to what they say and play it cool.
Longman dictionary of contemporary English. 2004.