Meaning
I have no more information about the answer than you do — I genuinely don’t know.
It’s a frank, slightly informal way of admitting total uncertainty rather than pretending to a knowledge you don’t have.
Origin
The phrase is a straightforward colloquial construction, comparing two guesses as equally uninformed rather than referring to any technical or historical practice. It became a common conversational stock phrase in American English by the mid-20th century, used whenever someone is asked a question they have no real basis to answer, and it has since spread widely into British and other varieties of English as an everyday, easily understood idiom.
Examples
A: Why did the server crash? B: Your guess is as good as mine.
Nobody at the office knows what happened — your guess is as good as mine.
A: When will the flight land? B: Your guess is as good as mine at this point.
As for who’ll win the election, your guess is as good as mine.
Frequently asked questions
What does 'Your guess is as good as mine' mean?
I have no more information about the answer than you do — I genuinely don’t know.
It’s a frank, slightly informal way of admitting total uncertainty rather than pretending to a knowledge you don’t have.