Green’s Dictionary of Slang

a contemptible person, esp. if superficially flashy; a fool; usu. referring to a smalltime gambler.

1887		F. Francis Jr Saddle and Mocassin 228: There they were, sir, playing poker [...] Each tin-horn with the most profound contempt for the others’ skill.
1893		Salt lake Trib. (UT) 18 July 8/3Another trck of the tinhorn is to watch around a faro table until he sees a bet which the owner has overlooked and which the tinhonr pounces upon: .
1897		O. Wister Lin McLean 253: Bull-whackers, cow-punchers, mule-skinners, tin-horns!
1909		R. Service ‘The Ballad of Hard-Luck Henry’ Ballads of a Cheechako 75: The smooth Beau Brummels of the bar, the faro men, are there; / The tinhorns and purveyors of red paint.
1917		Boy’s Own Paper XL 2 65: ‘I’m a piker!’ he cried, ‘a chechako and a kid and a tin-horn and a piker!’.
1922		S. Lewis Babbitt (1974) 23: Those tin-horns that spend all they got on dress-suits and haven’t got a decent suit of underwear to their name!
1933		‘Paul Cain’ Fast One (1936) 186: I don’t like the racket [...] It’s full of tin-horns and two-bit politicians and double-crossers.
1940		O. Strange Sudden Takes the Trail 17: A cheap tinhorn, lets the other fella win till he’s too pie-eyed to notice crooked play.
1945		J. Farnol Piping Times 225: You’re just a tinhorn, a yellow, four-flushing, spineless quitter.
1951		R. Service ‘Dumb Swede’ in Lyrics of a Low Brow 19: They reckoned they were mighty slick, / Them two tinhorns from Idaho.
1953		S. Bellow Augie March (1996) 81: Tinhorns and small-time racketeers.
1959		N. Nye Long Run (1983) 170: That tinhorn won’t earn the damned grub he swallers!
1962		(con. late 19C) E. Lucia Klondike Kate 67: The early tinhorns and slickers hadn’t realized yet what a giant bonanza they were sitting on.
1968		A. Steinberg Sam Johnson’s Boy 440: You vote like you damn please, Lyndon, and I’ll vote like I damn please. You’re just a tinhorn.
1975		T. Berger Sneaky People (1980) 90: You’d order one soda and two straws: that’d be your idea of a date, you tinhorn.
1979		(con. late 19C) J.T. Edson Gentle Giant 62: Now if that’s not just like a sneaky tinhorn!