on a knife-edge

phr

• ① (人)(对事情的成败)感到惴惴不安<about>
  • He's on a knife-edge about the examination.他对考试感到忐忑不安。

• ② (事情的成败)处于微妙的状态
  • The success or failure of the plan was balanced on a knife-edge.计划的成败尚未定局。