be out of your depth

Từ điển Oxford Advanced Learner 8th

be out of your ˈdepth idiom
1. (BrE)to be in water that is too deep to stand in with your head above water
• If you can't swim, don't go out of your depth.
2. to be unable to understand sth because it is too difficult; to be in a situation that you cannot control
•He felt totally out of his depth in his new job.
Main entry: ↑depth_idiom