It means “I couldn’t care less” : the speaker is saying they do not care at all about the topic. The phrase “I could care less” is often heard in casual speech too, but it literally suggests the person does care a little, so the standard, clearer form is “couldn’t care less.”

Plain meaning

  • I couldn’t care less = “I’m completely indifferent.”
  • It’s used to show strong lack of interest, annoyance, or dismissal.
  • In everyday conversation, people sometimes say I could care less with the same intended meaning, even though it is logically the opposite.

Example

  • “He can keep arguing; I couldn’t care less.”
    Meaning: “I do not care about this at all.”

Which version to use

  • Use couldn’t care less in writing, presentations, or anything more formal.
  • You’ll still hear could care less in informal American English, but it can sound ungrammatical or confusing.

TL;DR: It means “I don’t care at all,” and the clearest version is “I couldn’t care less.”