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1. No one knows (except for God). God knows how long it will take for my application to be processed. A: "How do you think you did on your exam?" B: "God knows!"
2. It is absolutely true, certain, or definite that; it is obvious or clear that. God knows I need to do well on this test or I can kiss my scholarship goodbye!
God 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: "God 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: "God knows that I've tried, but she is adamantly refusing." A: "Then try harder!"
Also, goodness knows; heaven knows.
1. Truly, certainly, definitely, as in God knows I need a winter coat. This expression, which originated about 1300 as God wot, does not necessarily imply that God is all-knowing but merely emphasizes the truth of the statement it accompanies. The variants using goodness and heaven are euphemisms that avoid taking God's name in vain. [Second half of 1500s]
2. Also, God only knows. Only God knows, that is, neither I nor anyone else knows, as in God knows where I've stored those photos, or God only knows how many people will join the march. [Second half of 1500s]
1 I do not know; no one knows: ‘What’s going to happen next?’ ‘God knows.’
2 used for adding emphasis to a statement, opinion, etc: God knows how he manages to survive on such a small salary. ♢ I’m no gardening expert, goodness knows!(Some people may find the use of God knows offensive.)