Bully
mid 16th century: probably from Middle Dutch boele ‘lover’. Original use was as a term of endearment applied to either sex; it later became a familiar form of address to a male friend. The current sense dates from the late 17th century.
wiktionary
From 1530, as a term of endearment, probably a diminutive ( + -y) of Dutch boel(“lover; brother”), from Middle Dutch boel, boele(“brother; lover”), from Old Dutch *buolo, from Proto-Germanic *bōlô (compare Middle Low German bôle(“brother”), Middle High German buole(“brother; close relative; close relation”) (whence German Buhle(“lover”)), Old English Bōla, Bōlla(personal name), diminutive of expressive *bō-(“brother, father”). Compare also Latvian bālinš(“brother”). More at boy.
The term acquired negative senses during the 17th century; first ‘noisy, blustering fellow’ then ‘a person who is cruel to others’. Possibly influenced by bull(“male cattle”) or via the ‘prostitute's minder’ sense. [1] The positive senses are dated, but survive in phrases such as bully pulpit.
etymonline
bully (n.)
1530s, "sweetheart," a term of endearment applied to either sex, of uncertain origin; perhaps from 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;" 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" might be "protector of a prostitute," which was one sense of bully (though it is not specifically attested until 1706). "Sweetheart" words often go bad in this way; compare leman, also ladybird, which in Farmer and Henley ("Slang and Its Analogues") is "1. A whore; and (2) a term of endearment." Shakespeare has bully-rook "jolly comrade."
The adjective meaning "worthy, jolly, admirable" is first attested 1680s, and preserves an earlier, positive sense of the word. It enjoyed a popularity in late 19c. American English, and was used from 1864 in expressions, such as bully for you! "bravo!"
bully (v.)
"overbear with bluster or menaces," 1710, from bully (n.). Related: Bullied; bullying.
