The expression "Bite the Bullet” used today means to go through the pain (physical or mental) and get on with it.
In the 19th century, it could literally mean to bite a bullet. At the time, there was no such thing as pain relief or anesthesia when soldiers were injured on the battlefield and needed surgery, including amputations. They might be given an alcoholic drink but they were given a bullet to bite down on to prevent them from screaming out loud.
Of course, it didn't need to be a bullet - a piece of wood or leather strap would work too - but bullets were readily available.
"Biting the bullet" is a metaphor used to describe a situation, often a debate, where one accepts an inevitable impending hardship or hard-to-refute point, and then endures the resulting pain with fortitude.
The phrase "bite on the bullet"That was first recorded by Rudyard Kipling in his 1891 novel The Light That Failed.
Evidence for biting an actual bullet rather than something perhaps safer and less likely to be swallowed is sparse. It is said that Harriet Tubman related having once assisted in a Civil War amputation in which the patient was given a bullet to bite down on.
Another origin story is that it evolved from the British expression "to bite the cartridge", which dates to the Indian Rebellion of 1857, but the phrase "chew a bullet", with a similar meaning, dates to at least 1796.
Modern audiences need to recognize that in the era of the origin of this phrase bullets were typically made of lead, a very soft metal, and would have been independent of any charge or cartridge.