To "bite the bullet" is to endure a painful or otherwise unpleasant situation that is seen as unavoidable. The phrase was first recorded by Rudyard Kipling in his 1891 novel The Light that Failed. It is often stated that it is derived historically from the practice of having a patient clench a bullet in his or her teeth as a way to cope with the extreme pain of a surgical procedure without anesthetic, though evidence for biting a bullet rather than a leather strap during surgery is sparse. It may also have evolved from the British empire expression "to bite the cartridge", which dates to the Indian Rebellion of 1857. In this version of the etymology, the idea of tolerating necessary hardship refers to the British wish that the sepoys would ignore any small presence of animal fat in their paper cartridges.

In philosophy, a more specific meaning of the phrase is to accept unpleasant consequences of one's assumed beliefs. Sound reasoning requires its practitioner to always sustain a consistent set of beliefs. This may involve accepting a disturbing belief that is a consequence of one's currently held beliefs. It may be disturbing because it is counterintuitive or has other disturbing consequences. Given a philosopher's currently held beliefs that he or she is not prepared to give up, he or she may have to bite the bullet by accepting a particular claim offered as an extreme case or putative counterexample.