What is the origin of the idiom "someone could not care less" and when to use it?

The idiom "someone could not care less" expresses complete indifference or lack of concern about something. It originated in the mid-20th century as an extension of the earlier phrase "could care less," which was often used sarcastically. The idiom is used to emphasize that there is no lower level of caring, making it clear that the speaker has no interest or investment in the subject at all.