The standard expression is couldn’t care less. It means someone cares so little that caring less is impossible. In other words, they are at the absolute minimum level of concern.
Example: I couldn’t care less about the final score. This clearly means, I do not care at all.
You may also hear could care less, especially in informal American English. It is very common in speech, but many people criticize it because the literal meaning seems backwards. If you could care less, that suggests you do care at least a little.
Because of that, couldn’t care less is the safer choice in formal writing and careful speech.
- Standard and clear: She couldn’t care less what they think.
- Common but often criticized: He could care less about the movie.
If you want wording that sounds precise and avoids debate, use couldn’t care less.