Green’s Dictionary of Slang

(US) an excl. used to express excitement or success.

[1868		Terr. Enterprise (Virginia, Nevada) 30 Sept. 3/2: When they thus got three beans in a row they were to call out ‘Keno!’ and rake in the pot [DA]].
1890		J. Jefferson Autobiog. 65: In the death scene, just as Richard expired, a voice, signifying that the game was over, shouted ‘Keno!’ [DARE].
1905		‘Hugh McHugh’ Get Next 64: ‘Have you a sort of nervous hesitation in your hunger and does everything you eat taste like an impossible sandwich?’ ‘Keno!’.
1919		Van Loan ‘The Bone Doctor’ in Score by Innings (2004) 365: ‘Keno!’ says Jones to us, ‘I’ve got him loaded on my litttle wagon.’.
1922		P.A. Rollins Cowboy 81: The throwing of an elusive steer, the breaking of a whiskey bottle, the being thrown from a horse’s back, each might evoke ‘Keno!’.
1931		C.E. Mulford Hopalong Cassidy and the Eagle’s Brood 139: ‘I say keno!’ answered Dave [...] ‘Four dead men in two nights!’.
1940		L. Short Brand of Empire 181: Ben says your boys have called the turn. The next time I see you, it’ll be keno.
1968		R.F. Adams Western Words (2nd edn) 169/1: keno. A cowman’s word meaning that everything is all right. The satisfactgory conclusion of any act might evoke the exclamation, ‘Keno!’ From the gambling game.