If you cringe whenever you hear "I could care less," you'll be glad to know that someone has finally corrected this statement. It was on last night's Modern Family when one of the characters - the rather uptight Mitchell - corrected his partner who had said emphatically, "I could care less."
To express a complete lack of concern, you'd say instead, “I could not care less.” If you could care less, it means you still care. It means that there is still a reservoir of care left within you. If you could not care less, it indicates that you have not one smidgen of care left to give.