所谓的 “到刀子下面 to go under the knife” 意思是动手术，接受手术治疗。

例句

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.

I had tonsillitis eight times before doctors decided that I needed to go under the knife and have my tonsils removed.

请注意

不要把这个短语同 put to the sword 混淆，后者的意思是杀戮。

The captured soldiers should have been kept in a military prison but instead they were put to the sword. Hundreds were killed.