Green’s Dictionary of Slang

1. the ultimate solution, the culmination.

1702		Cibber She Would and She Would Not I i: Well said again, that was a Clincher.
1754		B. Martin Eng. Dict. (2nd edn).
1785		Grose Classical Dict. of the Vulgar Tongue n.p.: A clincher, or to clinch the nail; to confirm an improbable story by another: as, a man swore he drove a twopenny nail through the moon, a by stander said it was true, for he was on the other side and clinched it.
1795		J. O’Keeffe Life’s Vagaries 10: A man’s last will is the clincher.
1812		Spirit of Irish Wit 214: So the clencher by unanimous consent won the bet.
1821–5		‘Bill Truck’ Man o’ War’s Man (1843) xiv: I had but two objections; but, mind me, they were clinchers.
1832		in Egan Bk of Sports 104: death comes but once, the philosophers say, / And ’tis ture, my brave boys, but that once is a clencher.
1833		W. Carleton Traits and Stories of Irish Peasantry III 40: ‘Go on, Denny,’ they would say again [...] ‘Stick to him! – very good! – that’s a clincher!’.
1843–44		Dickens Martin Chuzzlewit (1995) 358: Oh but it was a clincher for the British Lion, it was!
1859		G.A. Sala Twice Round the Clock 286: The pre-ordinary clincher had been the erection of the hideously lugubrious penitentiary.
1865		Leaves from Diary of Celebrated Burglar 147/1: This was a clincher and I could contain no longer the laugh that was in me, so out it came.
1873		Sportsman (London) ‘Notes on News’ 27 Dec. 4/1: This last is simply a ‘clincher’ for the Italian opera; and Covent Garden and Drury Lane may in future hide their heads.
1878		F.H. Hart Sazerac Lying Club 193: This was the clincher.
1885		Bulletin (Sydney) 12 Sept. 6/2: Father Slattery replies by telling the Bishop that if ‘he cannot see the promise of the Church to Peter, and the constitution of his Church for all time, the absolute necessity of an infallible head,’ he can neither ‘help him nor hope to convince him.’ This of course is a ‘clincher.’.
1891		A. Day Mysterious Beggar 270: He came near forgettin’ th’ clincher I mos’ wanted.
1895		J. Caminada Twenty-Five Years of Detective Life I 67: This was a ‘clencher’ for them.
1900		Marvel XIV:344 June 3: The argument or explanation was generally accepted as a clincher.
1904		Sydney Sportsman (Surry Hills, NSW) 1 June 3/3: [A]nd as if to give the jewellery man a clincher, the bushie fairly yelled [etc].
1909		Gem 23 Jan. 13: ‘But I shall insist,’ said D’Arcy, evidently regarding that as a clincher.
1945		J. Hoyt Cummings Fatal Pay-off 59: If a lab test shows it matches with Reali’s blood [...] that’ll be the clincher.
1955		B. Schulberg On the Waterfront (1964) 13: And then comes the clincher.
1957		R. Prather Always Leave ’Em Dying 14: I’d known enough about the guy to hate him even before I’d met him, but meeting him was the clincher.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 216: That last sentence was the clincher for me.
1969		‘Iceberg Slim’ Pimp 249: The clincher was this broad’s wide-legged walk.
1977		E. Bunker Animal Factory 128: That’s the clincher.
1989		H. Leonard Out After Dark 37: And then the clincher: ‘Sure why else would he write it?’.
1997		D. Farson Never a Normal Man 132: After three days he vanished. This was the clincher.
1999		Indep. Rev. 16 Aug. 4: That was the clincher.
2000		I. McDowall A Study in Death 267: The bottom line, the clincher, had been the prospect of media attention.
2015		S. King Finders Keepers (2016) 154: The caller ID thing was the clincher.

2. an irrefutable lie.

1864, 1867, 1870		Hotten Sl. Dict.
1895		Crowe Aus. Sl. Dict. 18: Clincher, a big lie.