If you could care less, then at least you care some.
Saying "I could care less" is American slang that's been around since the 1960s.
It's not meant to be taken literally.
It has the same kind of implied opposite meaning as when people sarcastically say "Yeah, right" or "Oh, sure" or "Uh huh, I really believe that!" or "I should be so lucky!" (while rolling eyes),
It probably comes from the 1950s expression, "I suppose I could care less, but I don’t see how", and got shortened along the way.
In any case, it's even in the Oxford dictionary as an American colloquialism, so leave its users alone