bullet (n.) Look up bullet at Dictionary.com
1550s, "cannonball" (a sense now obsolete), from Middle French boulette "cannonball, small ball," diminutive of boule "a ball" (13c.), from Latin bulla "round thing, knob" (see bull (n.2)). Meaning "small ball," specifically a metal projectile meant to be discharged from a firearm, is from 1570s. Earliest version of the figurative phrase bite the bullet is from 1891, probably with a sense of giving someone a soft lead bullet to clench in the teeth during a painful operation.