"What you call meaning is what I call post-hoc rationalisation. What "I couldn't care less" means is that I am ... It says nothing about how much I care, only that I'm unable to vary the amount in a downwards direction."
Actually, it can mean either, and perhaps this
ambiguity is one more reason why attempts to
sort out the evolution of the various meanings
has become such a muddle.
In an example cited earlier, a fiction writer wrote "he wished he could care less (about someone)." In that sentence, "could care less" means being capable of modifying the amount of caring. In that sense,
the phrase could be made negative without changing its literal meaning: "But he couldn't care less." That would mean, in that context, exactly what you said above: that he lacked flexibility in the amount of care that he was able to provide.
However, there is another meaning, equally
literal and logical, that would be called for in
a different context, where "I couldn't care less"
means "I care so little that it would be impossible for me to care less"; i.e., I care not at all. Some would consider this an idiom, perhaps because
they see it as foreign to their sensibility, but to me it is as straightforward as the other meaning
given above. It is the context that provides the
clue as to which interpretation is called for.
If you need an example, think of the phrase
"it couldn't get any worse." There are two
possible interpreations:
1. It is so bad that worsening is impossible.
2. Worsening is extremely unlikely.
Or the phrase "I couldn't wish for more."
1. There is simply nothing else I could wish for.
2. I am prevented by circumstances from wishing for more.
"Have you considered that "I could care less" might have quite deliberate, an ironic send-up of the circumlocution of "I couldn't care less", which then got copied by someone who didn't recognise it for what it was? Irony's like that."
That's how I see it, too.
Michael West
Melbourne, Australia