Uncertain or unknown. Why dogs and cats hate each other is anybody's guess. No one has been able to determine why my daughter has been having nightmares—apparently, it's anybody's guess.
Something that no one knows for sure, as in Will it rain next Sunday? That's anyone's guess. [Mid-1900s] Also see your guess is as good as mine.
anyone's guessa totally unpredictable matter. informal
1999 Jason Elliot An Unexpected Light The most likely scenario was a government alliance with the forces of the north, although it was anyone's guess how long such a Faustian pact might last.