A knife is a tool for cutting or a weapon and consists of a flat piece of metal with a sharp edge on the end of a handle.

To knife someone means to attack and injure them with a knife.

A surgeon's knife is a piece of equipment used to cut flesh and organs during operations. It is made of metal and has a very thin sharp edge.

If you go under the knife, you have an operation in a hospital.

If someone does something like a knife through butter or like a hot knife through butter, they do it very easily.

If you have been in a place where there was a very tense atmosphere, you can say that you could have cut the atmosphere with a knife.

If a lot of people want something unpleasant to happen to someone, for example if they want them to lose their job, you can say that the knives are out for that person.

If you twist the knife or if you turn the knife in someone's wound, you do or say something to make an unpleasant situation they are in even more unpleasant.