To go under the knife means to have an operation.
To go under the knife 就是说做手术。
Examples:
England's best footballer won't play at the World Cup. He's going under the knife and won't recover in time for the start of the tournament.
英国最好的足球运动员不会参加世界杯。他要做手术，不能及时恢复参加比赛。
I had tonsillitis eight times before doctors decided that I needed to go under the knife and have my tonsils removed.
