Cool itself, unless you are using it to mean mildly cold, is a slang word. Just because it is old slang doesn't mean that it is not still slang.

How can it be unsafe? Slang is just talking.

It is impolite and discriminatory to use slang names for people/a culture. I suggest that you forget it learning such things, so I refuse to tell you even though I know it. Think about how you want to be called by others before you call someone a slang name.

The Slang term for a fifty cent coin is a half a buck or 4 bits.

Coppers