Imagine a scale of 1 to 10 for how much you care about something. 1 being that you don't care at all and 10 that you care very much. If you couldn't care less, you'd have to be at the bottom of the scale already wouldn't you. You can't get a lower level of 'care'.
I could care less is American only, and actually makes the phrase mean the complete opposite, but it is still used in the same
I believe the negative is transferred (by what the tranformational grammarians call "negative raising") to some main clause which is then deleted (unspoken).
I could not care less. > I don't think/imagine that (, even if I tried,) I could care less. > I could care less. I could not care less. > There is no possible way that I could care less. (I alrea
There are some other phrases where the presence or absence of negation doesn't change the meaning. Each of these pairs of sentences mean the same thing.