I could care less
It means you don’t care. But if someone takes it literally, they’ll confuse you. It’s like saying you’re not very interested, but you still say you could be even less interested.
'I could care less about your stupid opinion,' said the kid who didn’t even know what it was about.
'I could care less about the homework,' said the kid who didn’t do it.
'I could care less about the test,' said the kid who didn’t take it.