I’ve always thought that phrasing became popular because it’s so easily said with a sneer: “I could CAAAAARE less,” while the correct phrasing doesn’t carry the same load of disdain: “I couldn’t care less.”
Entirely my own conjecture, backed up by no evidence other than hearing them said.
“My eyes! Ze goggles do nothing!”
Which is still close to the speech pattern we’re talking about.
True…along with “My Ferrari! I had to do awful things to pay for her!”
On a more classic note, how about “This old man, he played one…”?
Steven Pinker has a similar conjecture, but I’m not buying it. I believe that in general, per Occam’s Razor the simplest explanation is the most likely one. And the simplest explanation here is that some moron misheard “I couldn’t care less” as “I could care less” and started unthinkingly parroting it that way, and eventually, writing it that way. Thus we got “I could care less” from the same place that gifted us “could of”, “would of”, and “should of”.
Pinker, the author of optimistic tomes like “The Angels of Our Better Nature”, apparently doesn’t believe in morons. He works damn hard to justify every linguistic quirk and solecism as inherently correct, even if he has to jump through hoops to do it. Sometimes he’s actually right, but sometimes he stretches plausibility to the breaking point. Sometimes people just make mistakes and get things wrong.
I ain’t got the learnin’ to figure out ped … pedan … what you said. I’ll just scratch my head and git along as best I can. 
I think it’s more likely that the n’t got elided in quick speech, rather than any one person mishearing it. So it would have kinda gradually faded.
As I say it now, it sounds like koongkareless. And the ng is very soft. (If you know IPA, I say approximately [kʊŋʔkeɚlɛs])
Very likely, but it really amounts to the same thing and is probably a combination of mispronouncing and mishearing, as opposed to competing hypotheses that it’s the result of some clever linguistic creativity.
What—you don’t believe “I meant to do that!”?
I’m just baffled why you need to use pejoratives when describing linguistic evolution. Really, edit “moron” and “unthinkingly parroting” from the above quote to something neutral and you become reasonable. As you wrote it, you sound like an asshole ignorant about linguistics.
And also ignorant of social registers, because you’re misjudging the audience here. An educated, informal register is widely used here on the SDMB. Your prescriptivist rantings come across as uneducated and formal, which is the other half of the reasons why you’re getting pushback. (The first being your ignorance of linguistics.)
And here we return to that immaterial concept of “correct”.
The misheard spread. It was selected for. It was, in the evolutionary analogy, a fitter version able to outcompete “couldn’t.” Pinker hypothesizes why it was the fitter variant. Phrases that get repeated as chunks repetitively no longer function as individual words strung together, they are chunked functionally as a word. (This applies as well when monitoring kids’ language development. “Thank you” and “stop that” do not count as developing two word phrases; they are functionally just single words.) Within a word grammar rules need not apply; flow off the tongue and how it sounds/what is heard, are what matters.
Oh. To join the “moron” pile on. While it is of course true that 50% of people are of below average intelligence, it is as true that 50% are above average. It is basic math that a majority of people are not morons. 
Yes, but then you remove the whole reason for his interest in linguistics in the first place: to look down on other people.
That is a phrase I was ignorant of anyway! I looked it up and if I understand correctly that’s basically the academic way to label what I was calling “playing to the room”, yes? “Code switching” is contained within it too?
Then allow me to relieve your bafflement. I used pejoratives out of frustration with apologists like Pinker who seem to believe that all idiosyncratic language usage has a good and valid justification, and the only people genuinely making mistakes in language are “mavens” like Henry Fowler and William Safire. You do have a point, though, that a lot of pejoratives can be off-putting when making an argument – although this is, after all, the Pit.
Wut?? This is total bullshit. The only times I can recall when I’ve seriously criticized anyone’s writing on the Dope – other than the occasional jocular dig about something trivial – is when someone of truly ignorant and usually reprehensible views posts a screed that’s so badly written that it verges on illiteracy. You know the sort I mean – the sort of poster who’s usually banned shortly after. I’m perfectly fine with all sorts of colloquialisms on the board and use them myself, and you know that, or should know it. And I totally reject the idea that endorsing the value of standard English somehow equates to “ignorance of linguistics”.
That’s not Pinker’s argument. You’ve invented yet another convoluted explanation for a phenomenon that likely has a very simple cause. What Pinker claims is that “I could care less” is intentional sarcasm, and is intoned differently than the original expression to sarcastically convey the opposite of its literal meaning.
Not sure what you are disagreeing with? He is hypothesizing that the communication of sarcasm was what made it the preferred variant. Now I have no idea if he has any evidence to support that hypothesis or what that evidence might even look like, versus say, @BigT’s explanation. I personally go with @BigT’s. The point that matters is that people voted with their mouths and they voted that way for some reason(s). “Correct” is a nonsensical concept in this context. And yeah, those who believe that language is or is not “correct” to some Platonic ideal are morons.
I think it’s more that the phrase can be thought of in different ways rather than it was misheard. I know that’s the case with me. I got into a discussion about this with my middle school English teacher. I thought it was “I could care less” since it meant that I was ambivalent and was flexible in how much I could care. If I cared a lot, then “I couldn’t care less” because I cared so much it was not possible for me reduce my care. For instance, “I couldn’t care less about my daughter because I love her so much that I will always care a lot about her.” Or the opposite: “I could care less about watching the TV show Friends because I’m ambivalent about the show and only watching because everyone else wants to watch it.”
To my math brain, it’s like there’s a scale of caring from 1 to 100. I care for my daughter at 100 and will never go below that (I couldn’t care less about her = I couldn’t care less than 100 for her). But with Friends, I currently care at level 50 for watching it, but I’m flexible in my level of caring and it could go down to 25 or up to 75. (I could care less about watching Friends = I’m flexible about how much I care about Friends and could go lower).
Regardless, I don’t use the phrase because of this ambiguity and the inconsistency in how people say it. I prefer the clearer phrase “I don’t care”.
But it would easily be mathematically possible for you to care less than 100.
Yes, the counter argument is that in math terms “I couldn’t care less” is stating “My current caring value is zero. Since negative caring is impossible my caring value cannot be further reduced.” And the phrase is properly delivered with disdain in your voice.
Taking both versions to mean “I don’t care at all” I now think perhaps that a different folk etymology / survival of the fittest argument for this phrase’s history runs about like this:
“I couldn’t care less” is disdainful, whereas “I could care less” is sarcastic. And for a generation raised on TV sitcoms, disdain is very weak sauce and sarcasm is the good stuff.
So the sarcastic version overtook the disdainful version in commonplace usage by commonplace folks.
I’m disagreeing with your description of Pinker’s argument. He is not claiming that there’s any “preferred” variant. You are. He’s not. He’s just trying to explain (wrongly, IMHO) how the variant came about. And, in fact, according to Google Ngram, the original “couldn’t care less” form is nearly 2.3 times more prevalent than the one you claim is “preferred”.
Except it didn’t. The original still prevails by a wide margin.
But the people who say “could care less” don’t as a rule emphasise “could” which would make sense in the sarcasm argument. It is mainly said with no stress to any particular word, which makes it nonsense parroted by idiots not any form of clever variant.