on the fence。Fence就是“篱笆”。On the fence的意思也就是“骑在篱笆上，左右不定”

　　例句:

　　Politicians are past masters at sitting on the fence, hedging their bets, and never giving a straight answer.

　　一般的政治家都是事后清，他们往往是墙头草，见风驶舵，谈问题向来不直接了当。