an expression that idiots use when they don't care about something or have no interest in something. what they really mean is "i couldn't care less"
john: this video game is so good. i could play it forever
jim: i could care less. i hate video games.
john: wait, so you could care less.... that means that you care somewhat. you mean "i couldn't care less"?
jim: what? no. i don't give a shit about your dumb video game.
john: you're a fucking retard.