God knows

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!" There's a pipe leaking? Yikes, God knows how long that's been going on!

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! What are you, a fool? God knows they don't let freshmen geeks like you into their club. Now that there are criminal charges against that man, God knows he'll never work in this town again!

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!" God knows I've tried—I just can't seem to get my average in Calculus above a D!

God knows

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]