If you think you’re cool, then it’s almost certain that you’re not!

Defining cool

cool/ku:l/

adj. cool·er, cool·est

Neither warm nor very cold; moderately cold: fresh, cool water; a cool autumn evening.
Giving or suggesting relief from heat: a cool breeze; a cool blouse.
Marked by calm self-control: a cool negotiator.
Marked by indifference, disdain, or dislike; unfriendly or unresponsive: a cool greeting; was cool to the idea of higher taxes.
Of, relating to, or characteristic of colors, such as blue and green, that produce the impression of coolness.
Slang
a. Knowledgeable or aware of the latest trends or developments: spent all his time trying to be cool.
b. Excellent; first-rate: has a cool sports car; had a cool time at the party.
c. Acceptable; satisfactory: It’s cool if you don’t want to talk about it.
Slang Entire; full: worth a cool million.

adv. Informal In a casual manner; nonchalantly: play it cool.

American Heritage Dictionary

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.