I don't give a shit
You're just not interested. You could also say 'I don't give a fuck.' It's the same thing, just more intense.
My boss said I had to do more work. I said, 'I don't give a shit.'
My friend asked me about my weekend. I said, 'I don't give a shit.'
My teacher asked me a question. I said, 'I don't give a shit.'