The phrase "could care less" has a logic error. It makes more sense for someone to say, "I couldn't care less" when the person wants to express that he doesn't care.
There's some suggestion that it comes from Yiddish style dialect. "I should be so lucky!", for another example.
One says, "although I care very little, there is some wiggle room to care even less". The other says, "I care so little, I couldn't care less than I do now".
Either way, I care little is the main message.
However, "couldn't care less" is more sensible, because what is the point of expressing that you care little, but still have room to care less? That sort of expression would only serve as a retort against an accusation that you do not care. ("A: You don't care at all! B: That is not true, I could care less.") B admits that he or she cares little, but objects to being characterized as entirely uncaring.
We should choose the expression based on its logical sensibility, rather than regional dialect.
I don't know about "could care less" conveying the semantics - I genuinely paused when I first read it to work out which meaning it had.
Let's saying "caring" goes 0 to 10. "Could care less" includes everything from 1 to 10, assuming integer granularity - it's very much the right hand side of the scale, anyway.
Only "couldn't care less" covers that 0 rating.
So - perhaps it's terrible in conveying semantics and only context/tone imply the disinterest being communicated?
Fun can usually be substituted freely for a word like 'entertaining'. Is there something wrong with saying "That was so entertaining"?
"Entertaining" is an adjective and you can use "so" as a modifier to express how entertaining whatever you were doing or watching is or was.
This probably explains it better than I can:
I agree, there's definitely two conflicting usages here: and activity can be the noun "fun", in the same way that doing something can be can be "bliss" or "hell" or "a complete waste of everyone's time". And so while I can say "My drive in to work was hell", I can't say "My drive in to work was so hell" (because not an adjective). But if I can say "My drive in to work was enjoyable", and even "My drive in to work was so enjoyable", then what's wrong with saying "My drive in to work was so fun"?
So, instead of "so fun", some people would prefer that you write e.g. "so fun that I squealed with glee", or even just "it was incredibly fun".
At least that's the argument I've come across. To the extent I even think about such things, I couldn't really care less if people use "so" like this; the meaning seems perfectly clear.
Millions of people use this idiom completely intuitively, following their instincts of language that I, as someone who doesn't have English as their native language, can only be jealous of.
"I could care less" is either meant to be taken sarcastically, or it came to be because the "dn't c" consonant cluster is hard to pronounce, the same reason you don't pronounce the 'l' in could, the 'k' in knee, knight or knave, the 'p' in psychologist or pneumonia, or half the letters in Wednesday.