There is a phrase native English speakers use that does not seem to make sense. They express their contempt and lack of concern for something by saying, "I could care less". Technically this should mean that they do care, at least a little.
This commonly repeated phrase began as “I couldn't care less” and is originally the title of the famous 1945 book, by Anthony Phelps. The book tells the story of the humour and hardships of the author's experiences as an air transport auxiliary pilot in WWII.
About a decade later the phrase “I could care less” started being used as a sarcastic variation. The problem with this second version is that it was so catchy that some people repeated it without using a sarcastic tone of voice. Some native speakers started using the expression they had heard without considering the actual literal meaning of the words. Then it becomes an error.
It can be difficult for a second language learner to tell if someone is being sarcastic.
Sarcastic example: That is the most beautiful dress I have ever seen in my life!
Really means: That is a very ugly dress.
Sarcasm may be shown by using an unusual mocking tone of voice. Watch people's faces while they talk. When a speaker is being sarcastic they will often either exaggerate their expressions or have a reaction that does not match their statement. They may also lengthen words.
Sarcastic example: That cucumber and liver cake tastes sooooooo great.
Really means: Don't try the cake; it is very bad.
In a written story content needs to be used. If you read something that seems completely wrong, you might be reading sarcasm.
Sarcastic example: Tom comes to Hong Kong to relax on the beach. Typhoon signal 8 is hoisted. He says, “It is really wonderful that I remembered my bathing suit.”
Really means: It is too bad that we came to Hong Kong only to stay in our hotel room.
Very often sarcastic remarks are exaggerated. Such comments are presented as extremely and unrealistically good or bad. The easiest way to show written sarcasm is to state it outright. Example: “It is really wonderful that I remembered my bathing suit.”, he said sarcastically.
Sarcasm is an exaggerated false statement meant to express the opposite meaning. It is detected by the way in which the sentence is expressed and the ridiculousness of the sentence. The phrase “I couldn't care less” is standard and correct. The phrase “I could care less” is only correct if it is made sarcastically. Since so many people use it incorrectly I suggest sticking to the literal original “I couldn't care less”.