How MUCH less could you care? A little? A lot? If you could care less, but you don’t, then it’s likely that you really do care an unspecified amount…which is not at all what you meant to convey! However, if you find it possible to care so little, and the object of your [not] caring so unimportant and unworthy that you are able to hold it in no regard whatever–in fact, ignore it totally– except, of course, to mention your total disregard of it–then you may actually be unable to care less! And this is what you really meant to convey.
But because the vast majority of people in the USA, either from ignorance, laziness, or the wish to follow current trends, are heard to say (with a flippant air), “I could care less,” you parrot what everyone says, not realizing that it expresses the opposite of what you mean. “So what?”, you say. “Everybody knows what it means, which is that I really don’t care. Nobody cares, man”…except for the few of us who still have respect for the English language and take pride in expressing ourselves correctly.
Oh……the correct way to say it is: “I couldn’t care less!”