I know that "cool" being used as an adjective to mean impressive has been around for a very long time, but when I was a kid (as far as I recall) this meaning didn't come into my consciousness until after I had discovered Superman. I think I was probably ten or eleven before this kind of usage was something that I could employ. And even then, I wasn't sure if it was going to last, because there were adjectives like "groovy," "hip," "far out" that had a short shelf life and expired. I find it remarkable that "cool" is still cool.

I think that cool is a very temporary condition. Because something that's cool at a certain time is cool because it's in fashion. Which means as soon as the fashion changes that thing has become uncool. So for Superman to keep being cool, he keeps having to change to be what is cool now. And whenever we look back on the past trends, we must have very intimate knowledge of history to tell what was cool and what wasn't cool at a specific point in time.

Superman was racist toward the Japanese circa 1942-45, but that was cool with people at the time. So being cool can be a trap. If people try too hard to be cool, they end up following trends that in hindsight were very bad. "It's a fool who plays it cool by making his world a little colder."