From Longman Dictionary of Contemporary Englishbe anybody’s guessbe anybody’s guessNOT KNOWto be something that no one knows What she’s going to do with her life now is anybody’s guess. → guess
Examples from the Corpus
be anybody’s guess• It's anybody's guess where he's disappeared to.• Disenfranchised voters are believed to number at least 100,000 - though why the lenders want their business is anybody's guess.• And how many days and weeks and years made up a duration was anybody's guess.• Which came first is anybody's guess, but the two are now working together to make the patient even more ill.• It is anybody's guess as to how the sale will fare.• Whether the idea will catch on is anybody's guess.• How often, when and where women-only units would see active service is anybody's guess.• What the Spice Girls' eight-year-old girl fans make of this is anybody's guess.• Strangely, what may happen at the taking of the sample itself seems to be anybody's guess. 