Bite the bullet
Origin: Before the advent of ether, the first anesthetic, surgery was a pretty desperate and painful affair. With the patient (although victim might be more descriptive) fully conscious and feeling the pain. These early surgeries were typically limb amputations or the removal of some object lodged into the body such as a bullet or arrowhead.
A typical amputation consisted of the "surgeon" using a saw to hack off the unwanted limb. The skin was then pulled down over the stub and sutured shut. Amazingly, some of these patients survived, but certainly the success ratio was low. Note that poorly skilled physicians today are called "hacks".
Even after the advent of anesthetics such emergency surgery has had to be performed at times. Particularly in times of war when anesthetics may be in limited supply or unavailable.
To ease the pain the patient was given a couple of stiff belts of whiskey to numb the senses, then given a stick or lead bullet to bite down on as the surgeon went to work with knife and saw.
The bullet or stick was given to let the patient focus their energy and attention on the biting instead of the cutting and pain. It may also have helped to reduce the screaming, which probably benefited the surgeon and attendants.
Why bite on a bullet? Made of lead, bullets are malleable. Although quite strong they will actually deform somewhat when bitten hard. Hence teeth would not break as would likely happen from biting a stone for example. Bullets are also readily available in times of war, when this type of surgery is frequently called for. "Bite the bullet" may have originated in the civil war.
The patient who bit the bullet was cooperating with the surgery. Clearly this poor fellow saw the surgery as unavoidable and absolutely necessary. He had decided to "bite the bullet" and get on with the surgery.
"Biting the bullet" has been captured in countless war movies and westerns.
Thanks to Jane, Kyle, John Gold, Kevin Morefield, Frederick Blume, Laurie Lee, and Melchor Balaguer