Which is the correct way to convey that it would be impossible for one to care less about something?

Interesting fact:

There’s a funny word for cooling yourself down: flabellation. That’s right, on a hot summer’s day when you’re reaching for a fan, you are preparing to engage in flabellation. The precise definition of flabellation is: “the act of fanning a wound to keep it cool” but outside of these fine details it is an unbelievably silly sounding word.