Phrases Americans Say That Make Zero Sense to Non-Americans

Or "I could care less" when most of the world says "I could not care less". If you can't care less then obviously you can't go any lower about caring.

But the American saying is: "I could care less about that." but that means (say) "I'm currently caring at 89% but I could care less down to 43%." which negates the whole meaning of the saying.

Too bad the 'why' is not explained at all. Some etymology would help there.