African-Americans invented cool as a stylistic defiance against racism during World War
II. This is simply well-documented social, historical, and linguistic fact. To be cool in the 1940s referred to the ability to be relaxed in
one's own style in any environment, an act of courage and mental strength for any Black person during the Jim
Crow era. Whether meaning high praise ("Cool!"), vetting someone to others ("she's cool"), or referring to a relaxed state of mind ("I'm cool, man") -- our modern usages all began in
jazz culture.
Oh, you
don't gotta worry about him, he's one of us, he's cool.
That
guy over there did the right
thing, he's cool in my book.
She's not cool, better watch your back.