Many/most Americans say something like "I could care less about the World Cup".

British people say "I couldn't care less about the World Cup".

Both are saying they have no interest at all in the World Cup. I don't know why Americans phrase it that way.

To give a documented example, the lyrics of Teenagers by My Chemical Romance:

    They said, "All teenagers scare the livin' shit out of me"
    They could care less as long as someone'll bleed

I always assumed the American version was using "could" ironically. Now I'm wonder if is an unintentional neologism.

Ah, I'd heard that latter one, but I thought that was just a mistake in the sense of "could of". TIL!