Green’s Dictionary of Slang

risky, adventurous; unlikely.

1906		‘O. Henry’ ‘A Cosmopolite in a Café’ in Four Million (1915) 29: A few long-shot winners at the New Orleans race-track.
1912		G.R. Chester Five Thousand an Hour Ch. ii: He had seen long-shot horses raise false hopes before.
1933		W.R. Burnett Dark Hazard (1934) 160: Picking a long-shot winner in a hurdle race! That’s not brains, that’s luck.
1962		S. Longstreet Flesh Peddlers (1964) 335: A long-shot dream.
1989		M. Amis London Fields 170: It was more the atmosphere of longshot desperation.
1997		L. Davies Candy 134: We were down to Little Angelo, a long-shot number-three choice.
2020		(con. 1991-94) W. Boyle City of Margins 26: She knows the easy life is a long-shot dream.