A scathing I couldn’t care less is a great way to handle a put down. Recently, though, I received an email asking the difference between that phrase and I could care less. They seem to be used interchangeably, but in fact their meanings are quite different.
The website World Wide Words says this about the phrase: A bit of history first: the original expression, of course, was I couldn’t care less, meaning “it is impossible for me to have less interest or concern in this matter, since I am already utterly indifferent”. It is originally British. The first record of it in print I know of is in 1901, in a story published both in the Church Standard and the Sunday Magazine. It seems to have reached the US in the late 1940s and to have become popular in the latter part of that decade. The inverted form I could care less was coined in the US and is found only there. It may have begun to be used in the early 1960s, though it turns up in a written form only in 1966.
But the phrases have come to have the same meaning in modern colloquial usage. Some people think that slurring of the negative ending caused couldn’t to become could. I know that sloppy pronunciation occurs regularly with words like fattest being said like fattess, so I can see that couldn’t might have changed to could.
A person using this phrase is most often being sarcastic. The sarcasm changes the emphasis on the words. For example, you might say I couldn’t care less, but with the other phrase you would say, I could care less. If you were to emphasize could in the latter phrase, it would have its literal meaning: that you care.
So if you really want your listener to understand that you don’t care, say I couldn’t care less. If you don’t care, then it is impossible to care less. Only if you DO care could you possibly care less. Yes, this is one of those grammar things that bugs me. Therefore, I COULD care less.