Английский Этимологический словарь - bully (n.)

- 1538, originally "sweetheart," applied to either sex, from Du. boel "lover, brother," probably dim. of M.H.G. buole "brother," of uncertain origin. Meaning (perhaps infl. by bull) deteriorated during 17c. through "fine fellow," "blusterer," to "harrasser of the weak" (1653). The verb is first attested 1710. The expression meaning "worthy, jolly, admirable" (esp. in U.S. slang bully for you!) is first attested 1681, and preserves the original sense of the word.