Jack Nicholson is cool. That is an undisputed fact.
I think people are cool when they naturally stand out in a crowd for some reason. None of that trying
to be different crap. Someone who is relaxed and confident yet humble, and just goes about their business. Somewhat eccentric, and wearing a pair of black shades helps, too.
Some things I think are cool: Big Sugar, red corvettes, black shoes, 80's nostalgia, thigh-high pantyhose, Hitchcock blondes, capri pants, Miami Vice, people who smile to mostly one side (sort of like Cretchien... but NOT REALLY).
Cool is completely relative, but unfortunately many people worship trends and adopt other people's views of cool instead of finding their own.