Using the phrase “could care less” actually means that one does, in fact, possess a measurable level of caring, concern or interest of a given situation and is contradictory to the context in which that phrase is commonly used. The use of “could care less”, in fact, expresses that one does care, have an interest, or a level of concern about a topic or situation.
Attempting to convey a position that one has no interest or concern would require the use of the phrase “couldn’t care less”, which indicates a state of mind completely devoid of caring, interest or concern. If one cannot care less, the indication is a level at which one could not possibly care less because he/she already does not care at all.
Incorrect usage: “I could care less what he thinks…” – meaning one does care and possesses a measurable level of caring from which feeling could be subtracted.
Correct usage: “I couldn’t care less what he thinks…” – meaning one does not care at all and thus it would be impossible to care any less or subtract feeling where none exists.