Green’s Dictionary of Slang

1. the vagina, esp. when considered larger than average.

1842		Wkly Rake (NY) 30 July n.p.: wants to knowWhat Neil Gallagher means by a ‘horse-collar’.
1870		Cythera’s Hymnal 34: A cunt large horse-collared and wet.
1882		Cremorne III 78: A fat lump with bubbies as big as an Alderman’s arse and a cunt like a horse collar.
1888–94		‘Walter’ My Secret Life (1966) VI 1256: I stopped her progress to see the horse collar from behind. A great, heavy, pouting lipped article it looked.
1896		Farmer Vocabula Amatoria (1966) 155: Harnais, m. [...] 2. The female pudendum; ‘the horse collar’.
c.1900		‘Ramrod’ Nocturnal Meeting 86: Some women I have fucked [...] have had cunts like horse-collars.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 88: Cut the cackle and hop into the horsecollar.
1973		B.S. Johnson All Bull 233: My sister has a hole like a horsecollar.

2. (US) a zero, esp. in sport, thus a negligable individual.

1886		Nat. Police Gaz. (NY) 29 May 14/3: Last year Clarkson was a king and McCormick a horse collar in the eyes of the Chicago club management. Mack felt the sting but said nothing.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 42: horse-collar, n. Another name for the cipher when indicating the score of a game or the mark of a student.
1944		Post (Lanarks) 23 Apr. 6/3: Horse collar — zero.
1957		M. Avallone ‘Power the Ball Platewards’ in Super Sports 🌐 Both of the rival hurlers were handing out horse collars to the opposition.

3. the hangman’s noose.

1920

Bulletin (Sydney) 19 Feb. 8/1: There is only one ‘Case for the Turk’ and that is a pine coffin, though plain burial at midnight [...] where four roads meet with a stake driven through the deceased would be cheaper. Yours affectionately, Henry Horsecollar.

4. (Can./US) a clerical or man’s high collar.

1808		J.P. Hurstone Piccadilly Ambulator I : .
1922		Joyce Ulysses 693: He had a nice fat hand the palm moist always I wouldnt mind feeling it neither would he Id say by the bullneck in his horsecollar I wonder did he know me in the box.

5. (US) marriage, wedlock.

1956

‘James Updyke’ [W.R. Burnett] It’s Always Four O’Clock 89: She was afraid the whole thing would blow up [. . .] and she’d lose her big chance to put the horse-collar on Lover Boy.