Green’s Dictionary of Slang

cool n.2

also coolness
[cool adj.]

1. [late 19C+] temper, poise, composure, attitude to life and ability to deal with it.

2. [1950s–60s] (US teen) a temporary armistice between opposing street gangs.

3. [1960s+] (orig. US black) sophistication, the prevailing fashion.

4. [1970s] (US campus) a fashionable, drug-taking (or whatever is deemed relevant) young person, as opposed to a straight, conventional person.

5. [2010s] (US) constr. with the, the epitome of excellence.

In phrases

blow one’s cool (v.) [1960s+]

1. to lose control, to become nervous or angry.

2. to ruin one’s image, to discomfit oneself, to make an exhibition of oneself.

blow someone’s cool (v.)

[1970s] to inform upon.

hold one’s cool (v.)

[1960s] (US) to keep calm; to relax.

keep one’s cool (v.)

[1950s+] (orig. US) to remain calm, despite circumstances to the contrary.

lose one’s cool (v.)

[1950s+] (orig. US) to lose one’s dignity or self-possession, to lose one’s temper.