I could care less
It means you could be even less interested. But if you're stupid, it means you're already as uninterested as you can be. It's like saying you could be more tired, but you're already the most tired person ever.
'I could care less about your stupid game,' said the kid who was falling asleep on the couch.
'I could care less about the project,' said the kid who didn’t even do any work.
'I could care less about the teacher,' said the kid who didn’t even pay attention.