From etymonline.com:

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 (cf. Ger. buhle "lover"). Meaning deteriorated 17c. through "fine fellow," "blusterer," to "harasser of the weak" (1653). Perhaps this was by infl. of bull, 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).

Perhaps this helps?

Peace

moo