Green’s Dictionary of Slang

(gambling) a wild guess, an adventurous attempt, a slim chance.

1869		Leisure Hour May n.p.: Will teach the tyro when to take a long shot... when to save himself by timely hedging [F&H].
1889		H. Smart Long Odds II 53: ‘I ventured to tell him Lord Ranksborow had got most of the long shots about Damocles’.
1892		E.J. Milliken ’Arry Ballads 16: A jolly long shot.
1900		Sporting Times 2 June 1/1: We read that Simon Dale was going great guns at the finish. It is more correct to say that a long shot through a Cannon almost got there.
1905		H. Green Actors’ Boarding House (1906) 17: What? A long shot like that win?
1917		Boy’s Own Paper XL:4 212: It’s a long shot, son; what do yo’ say?
1925		A. Christie Secret of Chimneys (1956) 164: A long shot [...] A very long shot. But it came off.
1933		J. Spenser Limey 39: But the long shot came home.
1945		G. Fairlie Capt. Bulldog Drummond 79: Our only chance is for her to make a mistake, and lead us to her: a long shot too, but the only possibility.
1946		Mezzrow & Wolfe Really the Blues 87: One time he caught a couple of long shots and run his small bankroll up to a couple of grand.
1951		A. Garve Murder in Moscow (1994) 68: I agree it’s a long shot, but I’d sure like to hear what he has to say.
1958		F.J. Hardy Four-Legged Lottery 14: I’m just trying to pick a long-pop. Just invest a bob or two. [Ibid.] 114: He studied the form guide, looking for a long shot with a chance; no use to back anything else but a twenty to one pop now.
1965		R.L. Pike Mute Witness (1997) 81: I never worry about anything, except maybe long-shots.
1977		(con. 1949) J.G. Dunne True Confessions (1979) 305: ‘That sounds like a long shot,’ Barry Backer said. ‘Homicide investigations are a series of long shots,’ Fuqua said.
1981		J. Wambaugh Glitter Dome (1982) 137: It’s a long shot that Mr Wheels even saw anything.
1995		J. Cameron Vinnie Got Blown Away 49: I banged on the door, hope for a long shot.
2000		Indep. Rev. 23 Feb. 11: I don’t know what was a longer shot: a Broadway production or a London show.
2013		T. Peacock More You Bet 7: The language of gambling and racing permeated the everyday language of the people. [...] rather than saying ‘No,’ one might have said, ‘Not by a long shot!‘.
2018		G. Disher Kill Shot [ebook] ‘Bit of a long shot, isn’t it, mate? What if [etc]’.