your guess is as good as mine used to tell sb that you do not know any more about a subject than the person that you are talking to does 我和你一样不知道；我和你一样心中无数 A：What's he doing? 他在干什么啊? B：Your guess is as good as mine. 我也不知道。 A：Who's going to win? 谁会赢？ B：Your guess is as good as mine. 我和你一样不知道。