Your guess is as good as mine - Idiom

Meaning:

1.      To have no idea. Not know the answer to a question.

2.      This idiom is used when one doesn’t know the solution or answer to a problem instead of admitting “I don’t know the solution or answer.”

Example:

1.      I have no idea when Arun will arrive. Your guess is as good as mine.

2.      If you want to know which one is the best of these movies, well, your guess is as good as mine.