anyone'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.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.