adj. 1
Not showing emotion; calm and in control of oneself.
Be cool. There's no need to panic.
adj. 2
Fashionable; trendy; hip.
adj. 3
Very interesting or exciting.
I think astronomy is really cool.
v. 1
To make cooler, less warm.

Related words

Word family: coolchest cool-headed coolheaded coolly coolness coolroom cooler cooling
Phrases: lose one's cool cool off cool it cool one's jets cool box cool down cool bag cool one's heels

Forms

Past participle: cooled
e.g. cooled → chilled, refrigerated, damped, refreshed…

Cool or Calm?

Of mental conditions, one is calm who triumphs over a tendency to excitement; cool, if he scarcely feels the tendency.

J. C. Fernald, English Synonyms and Antonyms (1914)

Use in printed books, per decade from 1850 to 2019.

2 letters: in¹, ok²
3 letters: bad¹, hip¹
4 letters: easy², fine¹, okay², safe¹, neat¹, damp¹, phat
5 letters: fresh¹, quiet², tight¹, chill¹, sober², dandy², stoic², aloof², staid¹
6 letters: casual³, wicked², classy², trendy², chilly², serene², frosty², frigid², quench¹, sedate², gnarly², snazzy², modish
7 letters: awesome², distant², relaxed², stylish², refresh²
8 letters: moderate³, composed², glorious³, freezing², chilling², tranquil²
9 letters: happening³, wonderful³, excellent³, collected³, temperate², audacious³, apathetic⁴
10 letters: acceptable⁴, nonchalant³
11 letters: fashionable⁴, indifferent³, unconcerned³, refrigerate⁴
12 letters: presumptuous⁴
13 letters: imperturbable

The small number after each word is its number of syllables (ok² = 2 syllables).