It doesn’t matter one bit; describing total indifference. The term originated in Great Britain, probably in the late 1930s, and was very popular by the 1940s. It not only expressed bored indifference but, during World War II, bravado. An informal history of civilian ferry pilots during the war by Anthony Phelps was entitled I Couldn’t Care Less (1946). In the mid-1960s the term was unaccountably changed by some Americans to I could care less, possibly influenced by the locution I should care, also meaning “I don’t care.” It, too, is now a cliché. Another variation recorded by Eric Partridge was I couldn’t care fewer, which, however, was short-lived.