Green’s Dictionary of Slang

1. a drink.

1821–6		‘Bill Truck’ Man o’ War’s Man (1843) 97: A muzzler of his superb Teneriffe, or a sparkler of his Madeira.
1873		Sl. Dict. 233: Muzzler a dram of spirits.
1895		C. Crowe Aus. Sl. Dict. 51: Muzzler, a glass of spirit.

2. a blow to the mouth or face.

1811		Lex. Balatronicum n.p.: Muzzler. A violent blow on the mouth. The milling cove tipped the cull a muzzler; the boxer gave the fellow a blow on the mouth.
1821		‘An Amateur’ Real Life in London I 350: He re-salated poor Pat with a muzzler, which drew claret in a moment.
1826		‘Nocturnal Sports’ in Universal Songster II 180/1: Had just tipped the cove o’ the ken a muzzler.
1834		Ely’s Hawk & Buzzard (NY) Sept. 6 n.p.: Sam popped in a tremendous muzzler with his left.
1848		Era (London) 6/36/3: Crockett [...] rattled away with both mawleys, and planted some muzzlers.
1873		Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 6: Muzzler - A blow in the mouth.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1895		C. Crowe Aus. Sl. Dict. 51: Muzzler, [...] a blow on the mouth.

3. (US) a crook, a strong-arm robber.

1865		Rogues and Rogueries of N.Y. 112: ‘Muzzling’ — A New Dodge. [...] ‘Muzzlers’ stand on corners, apparently skylarking among themselves. Suddenly one of them falls down and rolls himself against the pedestrian, who also must tumble. The man [...] is soon on top of the [pedestrian] [...] and [...] places his hand upon his face, presses his nostrils together [...] and covers his mouth with his hand. [...] While this is done by one, the others go through the man’s pockets.
1866		Birmingham Eve. Post 29 Mar. 5/1: An American ‘institution,’ termed ‘muzzling’. The ’muzzlers’ stand at street corners ‘larking.’ A passenger approaches [...] in an instant the ‘muzzler’ is upon him.
1935		A.J. Pollock Und. Speaks n.p.: Muzzler, a crook who impersonates a police officer.

4. (US, esp. prison) a homosexual, spec. a fellator.

1928		C. Panzram Journal of Murder in Gaddis & Long (2002) 116: I have met every kind of a crook there is. [...] last but not least muzzlers and guzzlers.
1932		V.F. Nelson Prison Days and Nights 149: The oral copulators are variously referred to as ‘muzzlers’, ‘fairies’, ‘fags’, ‘pansies’, and the like; the passive participants in sodomy are called ‘punks’, ‘gonsils’, ‘mustard pots’, or even more direct physical terms are used.
1934		G. Foster Lust 101: Say, I always know when I’m hungry, and I get somthin’ to eat pretty quick. There’s always at least one muzzler in de Tea House, an’ I know how to make their tongues hang out.
1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 31: muzzler (n.): A homosexual, as used by male prostitutes in a derogatory sense. Perhaps derived from the same muzzling or nuzzling of a female’s breasts on the part of a heterosexual.

5. (US) a contemptible person.

1928		R.J. Tasker Grimhaven 109: ‘Why, you big muzzler!’ Jock snorted.
1934		L. Berg Prison Nurse (1964) 83: Aw gee, boss; how can you expect me to play ball with that muzzler.
1940		S. Longstreet Decade 240: The muzzler invited me to ride to Philadelphia.
1950		Goldin et al. DAUL 143/2: Muzzler. (P) A contemptible fellow, as an informer or a degenerate; an unpopular prison keeper or other official.