Green’s Dictionary of Slang

cool adv.

1. [early 19C+] (also coolly) calmly, in an unruffled manner.

2. [late 19C] askance, suspiciously.

3. (US black) very.

In phrases

play it cool (v.) (also play cool, pluck it cool)

[1940s+] (orig. US black) to act in an uninterested or disinterested manner, to control every emotion.

play it cool

[1950s–70s] (US) a phr. of farewell.

take it cool (v.) (also take it coolly)

[mid-19C+] to relax, to remain undisturbed by events.