Green’s Dictionary of Slang

(a) an idiot, a fool.

1927		‘J.M. Hall’ Anecdota Americana I 81: ‘He’s a horse cock.’ [...] ‘Don’t call him that. He’s really a nice fellow.’.
2019		Twitter 22 Jan. 🌐 Oops! After such a strong start to the series, Reykjavik well and truly buggered it up in game two. Rocketing along at eight and over only works if you last more than 10 overs, guys. What a bunch of horsecocks.

(b) nonsense; thus horsey cocky adj.

1925		E. Hemingway letter 5 Jan. in Baker Sel. Letters (1981) 143: Don said it was all horse cock.
1934		H. Roth Call It Sleep (1977) 308: Aaa, he’s full of hoss-cops – C’mon, Polly!
1942		H. Miller Roofs of Paris (1983) 109: She was here teaching the students at the Academy of Design to draw swastikas ... the old primitive horse cock.
1953		W. Eyster Far from the Customary Skies 82: Baloney [...] Horsecock, Oakie.
1989		(con. 1960s) M. Kingston Tripmaster Monkey 41: Madam Orientalia knocking out horsey cocky locky astrology.

(a) (US) a sausage, salami.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1961		(con. 1945) G. Forbes Goodbye to Some (1963) 232: You’ll all be home eating fried horse cock in the morning.
1970		E. Thompson Garden of Sand (1981) 304: Get the kid a horsecock and cheese sandwich and some moo juice.
1977		J. Langone Life at the Bottom 111: The menu is cold cuts – the Navy calls it horse cock.
1989		J. Morton Lowspeak.
1991		O.D. Brooks Legs 25: A hunk of horse cock – that’s bologna.
2003		McGill Reed Dict. of N.Z. Sl. 106: horse cock Luncheon sausage roll.

(b) a gun.

1970

D. Ponicsan Last Detail 39: ‘Whoa there, Sunshine,’ says Billy. ‘We’re on our way, peace and quiet, law and order. You can take your hand off the horsecock you’re holding under the bar. You know the three of us would be able to cream you anyway.’ ‘How do you know I don’t have something that barks here and bites over there?’ asks the barman [...] ‘Ho, ho, ho, this redneck’s talking about firearms’.

(c) (US) a large penis.

[1680		School of Venus (2004) 42: Great horse Tarses hurt and open a Cunt too wide].
1927		‘J.M. Hall’ Anecdota Americana I 80: Walking down the street a couple of fellows passed a third, a mutual acquaintance. ‘Do you see that guy?’ asked one. ‘He’s a horse cock.’.
1990		R. Campbell Sweet La-La Land (1999) 99: He looked like a fucking dwarf with this horse cock in his hand.
2003		rev. of Man Academy 2 at www.gayvideodad.com 24 Apr. 🌐 Sizemore grinds Steele’s perky butthole into hamburger as he plugs it with [...] his own horsecock.