If you overhear someone say, "Man, that's a cold shirt," it doesn't mean the shirt wouldn't keep you warm. He's saying it looks good, cool.
Slang evolves at a furious pace, with key words dropping in and out of favor quickly, sometimes within seasons. Obviously, it's usually the "cool" people--from teens to musicians to students--who coin such terms.
But how do words get incorporated into slang?
"There's usually not a whole lot of logic to it, except that the people who coin these words use them as a way of excluding others--the uninitiated--or empowering themselves," says Jerry Cline-Bailey, associate professor of linguistics at Xavier University in Cincinnati. "If I can use language to befuddle you, then I have power over you."
With that in mind, check the following timeline for history's rendering of coolness.
1950s: boss, neat, hip,
early 1960s: gear, mod, groovy,
late 1960s: far-out, heavy, trippy, outta sight,
1970s: rad, superfly, funky
early 1980s: def, froody
mid-1980s: awesome, chill
late 1980s: funky fresh, hot, smokin'
1990s: dope, cold, fresh, fly, tight