Similarly, when my daughter says "Nice hat, Dad", she is not actually complimenting me on my choice of haberdashery, but rather, pointing out that she thinks it is not nice at all.
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Irony here refers to verbal irony, a discrepancy between the literally meaning of a phrase and its intended meaning, such as saying "What a nice day!" when it is raining.
Thus, "I could give a shit" and "I couldn't give a shit" are identical in meaning, as the former is doubtless intended ironically. Similarly for caring less.
Also, dropping the "not" in "Could not care less" is not reasonably said to be a grammar mistake. It's usually not sarcasm, either. Whether you agree that it's irony is a different matter (I'm pretty sure it's not).
I'm harping on about this because it's such a nice poster child for an entrenched (conventionalized) meaning of an entire expression as opposed to just a word, and for the lack of componentiality of meaning in language. In other words, there's more to the meaning of a sentence than just the meaning of its words. Componentiality is one of the points of debate between different schools of thinking in linguistics.
Only in some places - I can only remember having heard it on television from the US. I shiver in pain every time I hear it, too, so I'm pretty sure I haven't heard it in person (having lived in New Zealand and Australia).
Face it, people say "I could care less" for the same reason they say "then" instead of "than". There's just something about the English language that makes most of its native speakers unable to use it.
You came up with this irony theory because you've made the same mistake yourself, and your ego wants to deflect the accompanying shame.
The word "not" is not a particularly big word, but then again, most native speakers can't use "than" either.
It's amazing what an outburst of theorizing wankery your comment sparked.
http://en.wikipedia.org/wiki/Damn_with_faint_praise
I love David Mitchell, but this rant by other people is a long held irritation of mine: false pedantry:)