Green’s Dictionary of Slang

1. a fool, an idiot.

1862		in James Robertson Soldiers Blue and Gray (1998) 128: [In letters and diaries can be found such officer-references as] ‘horse’s ass’.
c.1888		Stag Party 12: ‘No,’ says the priest. ‘Yer not a horse but yer a horse’s arse .’.
1913		(con. 1871) Jocknick Early Days on the Western Slope of Colorado 22: Calling the agent by opprobious names, such as ‘Old Stiff,’ ‘Cabbage Head,’ ‘Horse’s Ass,’ etc.
1925		E. Hemingway letter 5 Jan. in Baker Sel. Letters (1981) 143: Don says not to be a horse’s ass and starve to death in a place with a name like Schruns.
1934		J.H. O’Hara Appointment in Samarra (1935) 120: He thinks Harry Reilly is a horse’s arse.
1935		E. Anderson Hungry Men 17: That bud of mine is a horse’s behind.
1952		C. Sandburg letter 20 May in Mitgang (1968) 481: I have known horse-asses who monkey-doodled with my copy.
1953		S. Bellow Augie March (1996) 464: Well, you horse’s foot [...] I guess you’re too dumb to make it.
1956		‘Ed Lacy’ Men from the Boys (1967) 21: Don’t make a horse’s rear out of yourself, especially if you might get on the force someday.
1957		‘Weldon Hill’ Onionhead (1958) 170: Doc had labelled him [i.e. a ship’s officer] as a real horse’s rectum.
1958		Southern & Hoffenberg Candy (1970) 68: ‘I know you must think I’m conservative, not up-to-date; perhaps a — a — ’ ‘ — a senile horse’s ass?’.
1966		‘Tom Pendleton’ Iron Orchard (1967) 38: That big ignorant horses’s-ass.
1971		T. Thackrey Thief 191: If I’d figured you were going to be such a horse’s can about it, maybe I would have blown the duke for you last night.
1974		C. Loken Come Monday Morning 6: He had to stop tryin’ to fix the tractor he broke tryin’ like a horse’s ass to buck through that big drift by the barn with the manure spreader.
1974		G.V. Higgins Cogan’s Trade (1975) 120: You horse’s cock.
1981		H. Gould Fort Apache, The Bronx 226: He’s the Commissioner, but he’s not a cop, and he doesn’t know what a horse’s ass you are.
1992		R. Price Clockers 320: If [...] we find out he once confessed for snatching Judge Crater, I’m gonna feel like a fucking horse’s ass.

2. a general term of abuse.

1886		Nat. Police Gaz. (NY) 19 June 14/2: The Boston baseball scribes [...] rip the stomachs out of their players when they lose, and laud them up to the heavens when they win. When they win they are daisys. and when they lose they are horses rosettes.
1886		Nat. Police Gaz. (NY) 27 Nov. 14/2: They are now daisies now but in the Spring when they come up alongside the old players they will find that they are only horse’s bouqnets.
1909–17		T.S. Eliot ‘Columbo & Bolo’ Inventions of the March Hare in Ricks (1996) 318: The queen she called him horse’s ass And ‘dirty Spanish loafer’ They terminated the affair By fucking on the sofa.
1952		W.R. Burnett Little Men, Big World 137: I don’t like to speak to you in this tone of voice, but you are the nearest thing to a horse’s ass I ever saw in clothes.
1953		L. Uris Battle Cry (1964) 49: This here guy is a horse’s ass.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 62: I want you to stop acting like Goddammned dreamy horse’s ass.
1982		G.V. Higgins Patriot Game (1985) 127: We all know what a gossip he is. In addition to being a horse’s ass.
1986		L. Heinemann Paco’s Story (1987) 157: They’ll fuck around and fart around and grab-ass around, and jack that thing up so it’ll be some horse’s ass of a hero’s statue.
2001		A. Schulman 23rd Precinct 151: If you write a lot of summonses—forty or fifty—are you a horse’s ass?