heaven knows

1. No one knows (except for God or the heavens). Often emphasized as "heaven only knows." Heaven only knows how long it will take for my application to be processed. A: "How do you think you did on your exam?" B: "Heaven knows!"

2. It is absolutely true, certain, or definite that; it is obvious or clear that. Heaven knows I need to do well on this test or I can kiss my scholarship goodbye!