be anybody's guess

be anybody's/anyone's guess

To be 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.
See also: guess
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

be anybody's guess

or

be anyone's guess

If you say that something is anybody's guess or is anyone's guess, you mean that it is impossible to know what is true or what will happen. How she managed to stay there for an entire month was anybody's guess. How long the fuel would last was anyone's guess.
See also: guess
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
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References in periodicals archive ?
One winds through these structures toward a colossal white zero painted on a black wall at the end of the room, in the center of which is an aperture leading to a destination that could be anybody's guess.