on a knife-edge

Từ điển Oxford Advanced Learner 8th

on a ˈknife-edge idiom
1. (of a situation, etc.)finely balanced between success and failure
•The economy is balanced on a knife-edge.
2. (of a person)very worried or anxious about the result of sth
•He's living on a knife-edge, waiting for the test results.
Main entry: ↑knife-edge_idiom