Green’s Dictionary of Slang

horse’s ass n.

also horse-ass, horse’s arse, ...behind, ...can, ...cock, ...foot, ...rear
[SE horse + ass n. (2)/arse n. (1)/behind n. (1)/can n.1 (1b)/cock n.3 (1)]

1. a fool, an idiot.

[US] in James Robertson Soldiers Blue and Gray (1998) 128: [In letters and diaries can be found such officer-references as] ‘horse’s ass’.
[US]Stag Party 12: ‘No,’ says the priest. ‘Yer not a horse but yer a horse’s arse .’.
[US](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.
[US]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.
[US]J.H. O’Hara Appointment in Samarra (1935) 120: He thinks Harry Reilly is a horse’s arse.
[US]E. Anderson Hungry Men 17: That bud of mine is a horse’s behind.
[US]C. Sandburg letter 20 May in Mitgang (1968) 481: I have known horse-asses who monkey-doodled with my copy.
[US]S. Bellow Augie March (1996) 464: Well, you horse’s foot [...] I guess you’re too dumb to make it.
[US]‘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.
[US]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?’.
[US]‘Tom Pendleton’ Iron Orchard (1967) 38: That big ignorant horses’s-ass.
[US]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.
[US]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.
[US]G.V. Higgins Cogan’s Trade (1975) 120: You horse’s cock.
[US]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.
[US]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.

[UK]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.
[US]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.
[US]L. Uris Battle Cry (1964) 49: This here guy is a horse’s ass.
[US](con. 1920s) J. Thompson South of Heaven (1994) 62: I want you to stop acting like Goddammned dreamy horse’s ass.
[US]G.V. Higgins Patriot Game (1985) 127: We all know what a gossip he is. In addition to being a horse’s ass.
[US]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.