Tim Bowen sheds some light on the origins and definition of the phrase to bite the bullet.
The Macmillan English Dictionary defines the expression to bite the bullet thus: “to force yourself to do something difficult or unpleasant that you have been avoiding doing”. For example, if you have toothache but have been avoiding going to the dentist because you are scared, the pain might finally become so great that you simply have to overcome your phobia, bite the bullet and visit the dentist.
The expression is generally believed to have its origins in the American Civil War when wounded soldiers were operated on in primitive conditions in field hospitals. When more reliable painkillers such as alcohol were not available, the next best thing was to give the unfortunate patient a bullet and tell him to bite on it as the surgeon’s knife went in. The patient was thus faced with a choice: he could die of his injuries or he could bite the bullet and face the excruciating pain of the amputation.