1530s, originally "sweetheart," applied to either sex, from Dutch boel "lover; brother," probably a diminutive of Middle Dutch broeder "brother" (compare Middle High German buole "brother," source of German Buhle "lover;" see brother, n.).Meaning deteriorated 17c. through "fine fellow" and "blusterer" to "harasser of the weak" (1680s, from bully-ruffian, 1650s). Perhaps this was by influence of bull (n.1), but a connecting sense between "lover" and "ruffian" may be in "protector of a prostitute," which was one sense of bully (though not specifically attested until 1706). The expression meaning "worthy, jolly, admirable" (especially in 1864 U.S. slang bully for you!) is first attested 1680s, and preserves an earlier, positive sense of the word.

1710, from bully, n.. Related: Bullied; bullying.