While I hope not to open the entire can of worms to which codyrobbins referred, I strongly suspect "I could care less" is a shorter form of the more obviously i

While I hope not to open the entire can of worms to which codyrobbins referred, I strongly suspect "I could care less" is a shorter form of the more obviously ironic "like I could care less". Treated as straight-up irony, the usage makes perfect sense.