Cool is a word that's pretty well known.
The weather could be cool meaning not very warm but not exactly cold.
More usually, cool is used to describe trendy or maybe cult.
That's a cool jacket you're wearing.
People can be cool too but just how cool?
Well, it looks like the coolest you can be is as cool as a cucumber! You know, the sort you put in salads or eat on cucumber sandwiches. Somehow it seems a strange collocation but here it is in a song of the same name.