Green’s Dictionary of Slang

1. a man hired by a brothel to act as a bouncer, strong-arm man, occasional lover or ‘husband’ of the madame or one of the prostitutes and a generally intimidating presence.

1693		Congreve Old Bachelor II i: Are you not a bully-back?
1721, 1726		N. Amhurst Terræ-Filius (2004) No. XXXIII 179: They have spiritual bravoes on their side, and old lecherous bully-backs to revenge their cause on every audacious contemner of Venus and her altars.
1762		Bridges Homer Travestie (1764) II 81: This said, the bully back of Troy / Stretch’d out his arms to take his boy.
1772		Bridges Burlesque Homer (3rd edn) 485: When this bully-back had gone, / ulysses found himself alone.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: bully back, a bully to a bawdy house, one who is kept in pay, to oblige the frequenters of the house to submit to the impositions of the mother abbess, or bawd, and who also sometimes pretends to be the husband of one of the ladies, and under that pretence extorts money from greenhorns, or ignorant young men, whom he finds with her.
1797		Bridges Burlesque Homer (4th edn) I 291: As to that thick-headed hang-dog, / Venus’s bully-back and bang-dog. [Ibid.] I 354: The bully-back of Troy / Stretch’d out his arms.
1811		Lex. Balatronicum.
1821		W. Scott Pirate II 293: Hold your peace, dear Dick, best of bully-backs, be silent.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a physically aggressive man, hired or otherwise, used as a bodyguard; also in fig. use, i.e. any form of supporter; occas. as v. (see cite 1728).

1728		Pope Dunciad I in Works III (1787) 82: Those brethren so lame and impotent, do ridiculously look big and very dull, and strut and hobble, [...] being led and supported, and bully-back’d by that blind Hector, Impudence.
1774		Caledonian Mercury 26 Sept. 2/1: Grant us but subsidies to stand the attack, / What should we fear — with you our bully back.
1784		J. Hartley Hist. Westminster Election 521: My friend, my bully-back, my right hand and left — my Æsculapian, leather-lung’d, Stentorian Orator, Churchill, arrived.
1806		Cobbett’s Wkly Political Reg. 13 Dec. n.p.: Gentlemen, this Mr Cobbett reminds me of a person in one of Congreve’s witty comedies called Bully-back, who is represented as attending and assisting Sir John Whittol in that play.
1807		Universal Mag. May 410/1: With Craufurd for thy bully-back, / What Windmills will ye next attack?
1834		Morn. Chron. (London) 5 Sept. 3/4: The smoke and dust which you [...] thought your bullybacks in Dublin would raise to cver your retreat.
1836		Dublin Eve. Post 6 Sept. 2/5: A Letter from Colonel Yorke, Private Secretary to the Lord Lieutenant, to Father Martin [...] the ‘bully-back’ of Father Dennis Delany.
1846		Dublin Eve. Post 25 Apr. 3/2: The only way in which his object may be acheved by a place-hunter, or by his bully-back at the press, is to kick the powers that be — to throw dirt at them.