English bully possibly influenced by bull ‘male cattle’ but not derivate from it. The word appeared in 1530s as ‘sweetheart’. The source of bully might be Dutch boel ‘lover; brother’, which probably is a diminutive of Middle Dutch broeder ‘brother’ (compare Middle High German buole ‘brother’, source of German Buhle ‘lover’). Meaning deteriorated 17c. through ‘fine fellow’ and ‘blusterer’ to ‘harasser of the weak’ (1680s, from bully-ruffian, 1650s). A connecting sense between ‘lover’ and ‘ruffian’ might be ‘protector of a prostitute’, which was one sense of bully (though it is not specifically attested until 1706) (Harper's Etymonline).