1550s, "cannonball" (a sense now obsolete), from 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 "do something difficult or unpleasant after delay or hesitation" is from 1891, probably with a sense of giving someone a soft lead bullet to clench in the teeth during a painful operation.
Beggars' bullets—stones thrown by a mob, who then get fired upon, as matter of course. [John Bee, "Slang," 1823]