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.
1. to lose control, to become nervous or angry.
2. to ruin one’s image, to discomfit oneself, to make an exhibition of oneself.
[1970s] to inform upon.
[1960s] (US) to keep calm; to relax.
[1950s+] (orig. US) to remain calm, despite circumstances to the contrary.
[1950s+] (orig. US) to lose one’s dignity or self-possession, to lose one’s temper.
see cool v.3 (2)