(the) Lord knows

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!" Kevin's always late, so Lord knows what time he will actually get here.

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! The Lord knows you can't leave your car here one minute longer than your meter allows, or you'll get a parking ticket. After two weeks off, Lord knows my email inbox will be chock-full. Ugh.

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.)