coolness

adj. cool·er, cool·est

v. cooled, cool·ing, cools

cool it Slang

cool (one's) heels Informal

[Middle English cole, from Old English cōl; see gel- in Indo-European roots.]

Synonyms: cool, calm, composed, collected, imperturbable, nonchalant
These adjectives indicate absence of excitement or discomposure in a person, especially in times of stress. Cool usually implies an alert self-possession, but it may also indicate aloofness: "Keep strong, if possible. In any case, keep cool. Have unlimited patience" (B.H. Liddell Hart)."An honest hater is often a better fellow than a cool friend" (John Stuart Blackie).
Calm suggests a serenity achieved through mastery over agitation or inner turmoil: "It was like coming across a bear in the woods: you were supposed to stand still and remain calm, against every impulse" (Cheryl Strayed).
Composed and collected stress self-control brought about by mental concentration: The dancer was composed as she prepared for her recital. The witness remained collected throughout the questioning. Imperturbable and unruffled suggest equanimity in the face of potentially disturbing circumstances: The crises of 1837 shook his previously imperturbable composure (James A. Henretta).
Nonchalant describes a casual manner that may suggest either confidence or lack of concern: "the nonchalant way of loggers with regard to injuries" (Molly Gloss). See Also Synonyms at cold.

Our Living Language The usage of cool as a general positive epithet or interjection has been part and parcel of English slang since World War II, and has even been borrowed into other languages, such as French and German. Originally this sense is a development from its use in African American Vernacular English to mean "excellent, superlative," first recorded in written English in the early 1930s. Jazz musicians who used the term are responsible for its popularization during the 1940s. As a slang word expressing generally positive sentiment, it has stayed current (and cool) far longer than most such words. One of the main characteristics of slang is the continual renewal of its vocabulary and storehouse of expressions: in order for slang to stay slangy, it has to have a feeling of novelty. Slang expressions meaning the same thing as cool, like bully, capital, hot, groovy, hep, crazy, nervous, far-out, rad, tubular, def, and phat have for the most part not had the staying power or continued universal appeal of cool.

American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.