Who-knows definition
Who knows? Maybe someday he'll finish writing his book and make lots of money selling it.
Since she hasn't studied at all I don't think she'll pass the test, but who knows? She might pull a Homer and actually pass; I doubt it though.
It could be one or the other, or both. Who knows?
Do you think I'll get married before I turn 30? - Who knows, you might never marry.