"Bite the bullet," or doing something unpleasant, comes from when soldiers would bite a bullet when they were being operated on without anesthetic.
“咬子彈”（bite the bullet），即做令人不愉快的事。這一習(xí)語來源于過去士兵們做手術(shù)不打麻藥，只能咬子彈忍痛。

There is some debate over whether or not this is true. The phrase has been in use since 1796 and has always meant to have a "stiff upper lip" before doing something you don't want to do.
關(guān)于這一來源是不是真的還存在爭議。這一短語從1796年就開始使用了，總是用來指在做不愿做的事情時咬緊牙關(guān)（have a stiff upper lip）。

It may have come from a belief that people can derive courage from biting a bullet, according to researchers.
研究者稱，這也許來自于一種信仰，認為人們能從咬子彈中獲得勇氣。