How many times have you heard someone say I could care less? For every time you’ve heard it, have you wondered why the person saying it COULD care less about something he seemingly doesn’t care about at all?
I could care less if you’re breaking up with me means that you’re admitting that you do care. And while we all probably do care when that happens to us, we usually don’t want the other person to know, so we say I couldn’t care less, meaning, the amount I care is the lowest possible amount there could be; it could not go any lower.
I could care less means that you actually could care less than you do, which means that you do, in fact, care. I couldn’t care less means that you could not care any less than you do.
That’s the correct usage—in case you care.