Word Origin & History
1530s, originally "sweetheart," applied to either sex, from Du. boel "lover, brother," probably dim. of M.H.G. buole "brother," of uncertain origin (cf. Ger. buhle "lover"). Meaning deteriorated 17c. through "fine fellow," "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 verb is first attested 1710. The expression meaning "worthy, jolly, admirable" (esp. in 1864 U.S. slang bully for you!) is first attested 1680s, and preserves an earlier, positive sense of the word.