Green’s Dictionary of Slang

1. (orig. US) a confrontation.

1895		Chicago Strike of 1894 32: Where I had to deal with the devil and wanted a show-down [...] I had enough sense to know that my only hope was to go into partnership with him [DA].
1897		Ade Pink Marsh (1963) 139: Englan’ got mo’ ships ’an us an’ mo’ soljahs, ’an’ might do ’iss country if it come to show-down.
1902		Ade Forty Modern Fables 81: The average Katy-Did had about as much Art Education as he could have Spread had it come to a Show-Down.
1913		J. London Valley of the Moon (1914) 54: He wades right in for a showdown, an’ nails Billy outside, before everybody, an’ reads the riot act.
1917		F. Packard Adventures of Jimmie Dale (1918) I v: I’ve got time now to set the stage for my own get-away before the showdown!
1922		E. Pound in Witemeyer Pound/Williams Correspondence (1996) 54: It is a show down. Those who don’t care 50 dollars a year for the arts, don’t care for MUCH.
1926		J. Black You Can’t Win (2000) 120: In the case of a show-down they will give up their valuables without a murmur because they are insured.
1930		J. Lait Put on the Spot 21: When it comes right down to a show-down, you hit ’em in the mush with a cream puff an’ you pick ’em up with a sponge.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 196: This was going to be a showdown with the old man, [...] he’d let him know that Studs Lonigan was going to be his own boss.
1936		R. Chandler ‘Guns At Cyrano’s’ in Red Wind (1946) 245: I’m in favour of a showdown, put them where they belong.
1944		D. Runyon Runyon à la Carte 39: The time comes for a showdown and [...] it is either her or old Em.
1952		H. McCoy Corruption City 67: When it comes to a showdown, he’ll pull a switch.
1965		H.S. Thompson letter 10 Aug. in Proud Highway (1997) 538: This is in fact a kind of showdown for me [...] In short, if I blow the action, I’m done.
1969		A. Buzo Rooted III i: We finally had a showdown when she caught me in flaggers with a bit of fluff.
1978		P. Theroux Picture Palace 247: My showdown with Papa.
1992		V. Headley Yardie 17: A little time to set himself up for the showdown.
1999		Guardian Guide 24–30 July 55: It even has a showdown atop a tower block.
2000		Guardian 8 Jan. 7: They want a showdown.

2. (US prison) an opportunity to break the prison rules without discovery.

1976

Edwardsville Intelligencer (IL) 30 Mar. 2/3: A ‘showdown’ is not of the High Noon variety. It means getting a good chance to do some forbidden act without being detected by a guard.