A sarcastic, ironic, irreverent take on the original phrase "couldn't care less." Just like with "couldn't care less," this phrase simply means that
one does not care. In this
case, it is intended as an ironic insult, since it prompts the other person to reason out that the only reason it would make sense to say "I could care less" instead of "I couldn't care less" is if they cared some and yet still extremely little. It'
s an insult delivered as a sarcastic compliment.
It's like if the TV is on really loud, and someone says, "can you turn it up?" That's a sarcastic comment on how loud it is. Just like "I could care less" is a sarcastic comment on how little you care.
Over
time, it became so commonplace that
people forgot it was sarcastic and just say it straight to
mean "I
don't care."