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!
Lord knows (that) I've tried
It is the absolute truth that I have tried my best (to do something). A: "You and Jeff just need to spend some time together and bond." B: "Lord knows I've tried. That boy just isn't interested in doing anything other than playing those damned video games." A: "You need to convince her to accept the deal." B: "Lord knows that I've tried, but she is adamantly refusing." A: "Then try harder!"
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
Lord (only) knows, the
Who knows? I certainly don’t. The locution that only God knows the answer to something dates from the eighteenth century or earlier. (It is also put as God knows and Heaven knows.) Defoe wrote (The True-Born Englishman, 1701), “Great families of yesterday we show, and lords, whose parents were the Lord knows who,” and a few decades later Swift wrote (Polite Conversation, 1738), “Lord of the Lord knows what.”
See also: lord