I'm not sure what you meant by this, but I don't think it's correct. I say "couldn't care less" and so do most people I know.
Really?! And you're a US native? May I then congratulate you on your erudition
I've come into contact with many US residents both in the US and elsewhere (I'm English, but with a cosmopolitan upbringing and work life), through both work and general friendship, and I can only ever remember them saying "I could care less" rather than "I couldn't care less" to convey their lack of interest in a given subject.
There are a few other more subtle "Americanisms" that have caught my eye over the years. They include the sometime elimination of "of" after "couple" (e.g. "I had a couple beers", rather than "I had a couple of beers"). To use "couple" without "of" sounds very strange to a UK-English speaker.
One other difference - which I think is far more subtle, is that way that American English uses "smells like" to mean something that UK-English speakers have two separate terms for. In UK-English, "smells like" strictly means that something has a smell that is evocative of something else, but that the "something else" isn't the source of the smell. In other words, if a UK-English speaker says "This blanket smells like pee", they don't mean that it actually has been peed upon - the word choice strictly implies that the blanket's smell resembles that of pee, but that the actual cause of the smell is non-specific. In UK-English, one would say "This blanket smells
of pee" to convey the impression that it has probably been peed upon.
That's one reason why so many UK-English speakers failed to understand the meaning of the Nirvana song title "Smells like Teen Spirit" (apart from the deodorant brand "Teen Spirit" not being on sale in the UK). The title comes from someone writing graffiti saying "Kurt Cobain smells like Teen Spirit" as a reference to his physical relationship with his then girlfriend who used Teen Spirit deodorant. In UK-English, we would have said "Kurt Cobain smells of Teen Spirit", and the song would have been different
PS: All the talk of peeing on blankets has left me wanting to watch The Big Lebowski again
