1. [mid-19C+] relaxed, calm, self-contained; often as imper., e.g. be cool!
2. [19C+] insolent, arrogant, impudent.
3. [late 19C+] (orig. US) of people, sophisticated, aware.
4. [late 19C+] good, fine, pleasing, admirable, a general positive sense.
5. [late 19C+] (orig. US) of a person, place or object, fashionable, stylish, chic, ‘with it’.
6. [1940s+] antonyms of hot adj.
(a) (drugs) not carrying or owning drugs, or believing that one has hidden them well enough to defy any search of one’s body or premises.
(b) (US Und.) not suspicious, either of people or objects.
(c) (US street gang) not carrying weapons or acting aggressively.
7. [1950s+] acceptable, satisfactory; esp. in phr. (that’s) cool, fine, ok.
8. [1960s+] comfortable, happy, on good terms.
9. [1970s] safe.
10. [1970s+] trustworthy.
[1950s] (US teen) a gang member, a 'hip' teenager; also attrib.
[1960s+] (S.Afr.) relaxed, in control.
see separate entries.
[1940s+] a sophisticated, competent, unruffled, able person; also attrib.
1. [mid-19C+] a cool, calm, controlled and competent individual.
2. [20C+] (US) a cool, unruffled demeanour.
1. [1960s+] a pleasant person.
2. [20C+] a calm, unflappable person.
[1960s–80s] (US black/teen) one who deludes himself into a belief that he is sense 3; in fact he is a jerk n.1 (3); thus do the cool jerk, to move in an ostentatious manner.
[1940s] (US black) a self-possessed, sophisticated and, as such, alluring man.
[1980s+] (US campus) something very new and appealing.
[1970s+] (US campus) an expression of approval.
[2000s+] (S.Afr.) a positive response to a question such as ‘how’s it going?’.
[1960s] (US teen) all wrong, whether socially, regarding fashion, work, amusements, etc.
[1980s+] (W.I./UK black teen) a phr. meaning everything is fine, all is going smoothly.
[2000s] (US prison) a general intensifier, usu. meaning truthfully or its opposite.
[1950s+] (orig. US) that’s satisfactory, that’s all right, don’t worry.
[1990s+] very fashionable, well-dressed, obsessively so.
[1980s+] (US teen) excellent! wonderful!
[1990s+] (US campus) an exclamation of approval, admiration.
SE in slang uses
[late 18C] a shroud; thus be put into one’s cool crape, to die.
[late 17C–early 19C] a female camp follower, specializing in selling brandy to the troops.
[late 17C–early 19C] cognac.
[1950s+] (orig. US) a bottle of beer.
[1980s+] (drugs) a smoke of methamphetamine.