"I could care less" is one of those idiomatic expressions, particularly in American English, that doesn’t necessarily mean what it says. There are many suggestions for the origin of the phrase, the most recent of which is that it's a corruption of "I couldn’t care less," possibly first used in the UK in the 1940s. By the 1960s, Americans had adopted the slightly modified version, perhaps out of laziness, poor hearing or deliberate irony.
Many contend it was laziness, much like the phrase “a hot cup of coffee,” changing to, "who wants a hot cup?" Most people would prefer to have a cup of hot coffee, or eat their cake and have it too. Simple reversals or omissions of words can result in someone saying that they could care less when they really don’t care at all.
There is some suggestion that the phrase "I could care less" may have been adopted because it fit into certain Yiddish phrases that deliberately mean the opposite and can be viewed as sarcastic. Such phrases include, "I should be so lucky," which really means that someone is not likely to have the luck. Another phrase, "Tell me about it," means the opposite, and it's merely a way to agree with the speaker. Alternately, speaking the term "Testify!" as used in certain Christian churches is a similar agreement that seldom means someone is actually going to sit down or stand up and give a testimony of how they converted to Christianity.
Another theory, advanced by linguistics specialist Henry Churchyard, suggested the statement "You know nothing and you care less," used in Jane Austen’s Mansfield Park, is the origin of the term. If this were the case, the "know nothing" would be comparative to caring less than the little you know. The current version of the phrase would then represent idiom by omission.
It should be stated that Mansfield Park is one of Austen’s least popular books, and was not well liked by the critics during Austen’s time and thereafter. That people would quote from it is in significant dispute, but if Austen used the term as one common to her day, it’s possible it was already in use. The whole quote, "You know nothing and you care less, as people say," is important because it advances the possibility the phrase was in use in Austen’s day and she is not its inventor.
In any case, "I could care less," is typically interpreted as not caring at all. Whether by omission, design, laziness or quote, it’s one of those mixed up idioms that plagues learners of English.