Definition: To accept the inevitable and proceed bravely.
Example: Despite the risk, John decided to bite the bullet and tell his landlady that he had sat on her cat.
The most widely expressed theory for this idiom is that it came from battlefield surgery, that patients in field hospitals were encouraged to bite down on a bullet during surgery to help deal with the pain and muffle their screams (nice.), but this seems unlikely when studied in further detail. There are no specific written accounts of this ever happening, and the choice of a bullet seems pretty poor. A leather strap or stick of wood would have been more effective, and less likely to be swallowed as well!
Another theory, also militarily grounded, is that it originated from the Indian Rebellion of 1857. When new rifles were issued to the Indian Sepoy fighters, they included greased paper cartridges that held the powder. These had to be bitten open. The Sepoys objected, not because it tasted horrible (which it probably did), but on religious grounds. The Hindu fighters feared the grease was derived from cows, and the Muslims feared it was from pigs. They were ordered to ignore their religious qualms, and thus ‘bite the bullet’.
Sounds great, doesn’t it? Alas, it seems the phrase predates this period of British colonialism.
So, we’re still no closer to nailing this one down.
Iddy doesn’t really care. He just wants to get the explosives out of his mouth!