cool

Etymology 1

From Middle English cool, from Old English cōl (“cool, cold, tranquil, calm”), from Proto-West Germanic *kōl(ī), from Proto-Germanic *kōlaz, *kōluz (“cool”), from Proto-Indo-European *gel- (“cold”). Cognate with Saterland Frisian köil (“cool”), West Frisian koel (“cool”), Dutch koel (“cool”), Limburgish kool (“cool”), German Low German köhl (“cool”), German kühl (“cool”). Related to cold.

adj

1. Having a slightly low temperature; mildly or pleasantly cold.
2. Allowing or suggesting heat relief.
   Linen has made cool and breathable clothing for millennia.
   Now that she had rested and had fed from the luncheon tray Mrs. Broome had just removed, she had reverted to her normal gaiety. She looked cool in a grey tailored cotton dress with a terracotta scarf and shoes and her hair a black silk helmet. 1963, Margery Allingham, chapter 2, in The China Governess
3. Of a color, in the range of violet to green.
   If you have a reddish complexion, you should mainly wear cool colors.
4. Of a person, not showing emotion; calm and in control of oneself.
5. Unenthusiastic, lukewarm, skeptical.
   His proposals had a cool reception.
6. Calmly audacious.
   Its cool stare of familiarity was intolerable. 1850, Nathaniel Hawthorne, “V. Hester at her Needle”, in The Scarlet Letter
7. Applied facetiously to a sum of money, commonly as if to give emphasis to the largeness of the amount.
   1900, Dora Sigerson Shorter, Transmigration You remember Bulger, don't you? You lost a cool hundred to him one night here over the cards, eh?
   My father was talking to the World's Fair Commission yesterday, and they estimate it's going to cost a cool fifty million. 1944 November 28, Irving Brecher and Fred F. Finklehoffe, Meet Me in St. Louis, Metro-Goldwyn-Mayer
8. (informal) Of a person, knowing what to do and how to behave; considered popular by others.
   He managed to conduct interviews with the least cool global figure – his father, Prince Charles – and the most cool, Barack Obama, in a way that allowed them both to look as good as they could. December 27, 2017, “The Guardian view on Prince Harry: the monarchy’s best insurance policy”, in the Guardian
9. (informal) In fashion and fancy, part of or befitting the most leading trends and habits of the in crowd; originally hipster slang.
   2008, Lou Schuler, "Foreward", in Nate Green, Built for Show, page xii The fact that I was middle-aged, bald, married, and raising girls instead of chasing them didn't really bother me. Muscles are cool at any age.
10. (informal) Of an action, all right; acceptable; that does not present a problem.
    Is it cool if I sleep here tonight?
11. (informal) Very interesting or exciting.
    I think astronomy is really cool.
12. (informal) (followed by with) Able to tolerate; to be fine with.
    I'm completely cool with my girlfriend leaving me.
13. (informal) (of a pair of people) holding no grudge against one another; having no beef.
    We're cool, right?

noun

1. A moderate or refreshing state of cold; moderate temperature of the air between hot and cold; coolness.
   in the cool of the morning
2. A calm temperament.
3. The property of being cool, popular or in fashion.

Etymology 2

From Middle English colen, from Old English cōlian (“to cool, grow cold, be cold”), from Proto-West Germanic *kōlēn (“to become cold”), from Proto-Indo-European *gel- (“to freeze”). Cognate with Dutch koelen (“to cool”), German kühlen (“to cool”), Swedish kyla (“to cool, refrigerate”). Also partially from Middle English kelen, from Old English cēlan (“to cool, be cold, become cold”), from Proto-West Germanic *kōlijan, from Proto-Germanic *kōlijaną (“to cool”), altered to resemble the adjective cool. See keel.

verb

1. (intransitive, literally) To lose heat, to get colder.
   I like to let my tea cool before drinking it so I don't burn my tongue.
2. (transitive, literally) To make cooler, less warm.
3. (intransitive, figurative) To become less intense, e.g. less amicable or passionate.
   Relations cooled between the USA and the USSR after 1980.
4. (transitive, figurative) To make less intense, e.g. less amicable or passionate.
5. (transitive) To kill.
   Maybe he would die. That would mean I had murdered him. I smiled, trying the idea on for size. One of the things that always had cheesed me a little was that I had no kills to my credit. I'd been in plenty of rumbles, but somehow, I'd never cooled anyone. Well maybe now I had my first one. I couldn't feel very proud of skulling an old man, but at least I could say that I'd scored. That was a big kick. 1965, "Sex Jungle" (narrated in Perversion for Profit)