What is the origin of the idiom "under the knife" and when to use it?

The idiom "under the knife" originated from the field of surgery, where the knife represents the surgical instrument used for incisions. It has been used metaphorically since the early 20th century to refer to undergoing a surgical procedure. It is used to describe the act of undergoing a surgical operation or procedure. It implies that someone is undergoing a medical intervention that involves the use of a surgical instrument, such as a scalpel.