1. No one knows (except for God). Lord knows how long it will take for my application to be processed. A: "How do you think you did on your exam?" B: "Lord knows!"
2. It is absolutely true, certain, or definite that; it is obvious or clear that. Lord knows I need to do well on this test or I can kiss my scholarship goodbye!
ˈLord (ˈonly) knows (what, where, why, etc.)...(spoken) used to say that you do not know the answer to something: She comes and goes as she pleases. Lord knows when we’ll see her again. ♢ ‘Why did she say that?’ ‘Lord knows!’(Some people may find this use offensive.)
See also: lord