"Your guess is as good as mine" is an idiom meaning "I don't know either," used to show you have no more information or insight than the other person when asked a question about something uncertain, highlighting mutual lack of knowledge. It's a way of saying you're equally clueless, often used when something is unpredictable, like when a delivery will arrive or if it will rain.
It is used when someone asks a question you can't answer, implying they likely can't either.
This idiom emerged in early 20th-century America, reflecting shared ignorance and uncertainty, with an early printed example appearing in a 1939 novel, *Waste Heritage* by Irene Baird.
While the exact moment is unknown, it grew out of casual language to describe situations of shared uncertainty, becoming popular in the US in the 1900s.
Example: If asked why the boss is upset, you might reply, "Your guess is as good as mine; I haven't heard anything".
