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Phrase(s): Your guess is as good as mine.
Inf. I do not know. • Jane: Are there any good movies playing tonight? Alan: Your guess is as good as mine. • Jill: How long should we bake this pie? Jane: Your guess is as good as mine.
McGrawhill's American Idioms And Phrasal Verbs
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I don't know any more than you do, as in As for when he'll arrive, your guess is as good as mine. [1920s]
American Heritage Idioms