Green’s Dictionary of Slang

1. a knockout; also fig. use.

1911		T.A. Dorgan in Zwilling TAD Lex. (1993) 53: Sleepy Mickey Finnegan who took a fine lacing for a round then put the k.o. on his man.
1913		Salt Lake Trib. 23 Feb. 27/1: Little ‘Kayo’ Brown finished flat on his back.
1915		N.Z. Truth 9 Jan. 6/1: [headline] Australian Scores K.O. in Round One.
1921		T.A. Dorgan in Zwilling TAD Lex. (1993) 51: Champion Has Two Sets To Kayo.
1930		W.R. Burnett Iron Man 26: Speed won by a K.O.
1939		W. Winchell ‘On Broadway’ 15 Nov. [synd. col.] When he wanted to be sure of a quick kayo, he’s put one [i.e. roll of nickels] in each fist.
1947		B. Schulberg Harder They Fall (1971) 8: Frank [...] had him on the verge of a kayo in the early rounds.
1955		B. Appel Tough Guy [ebook] [H]e clipped Milty the Poet on the side of the jaw, a stunner rather than a k.o.
1957		‘Ed Lacy’ Lead With Your Left (1958) 8: Ten amateur fights with nine kayos. Nine kayos, Mrs. Wintino.
1959		A. Sinclair Breaking of Bumbo (1961) 93: All we are is a lousy punch-drunk ex-champ, between a couple of real big men, jockeying around for the K.O.
1967		F. Salas Tattoo the Wicked Cross (1981) 28: A professional fighter with a string of kayos to his credit.
1968		S. Gore Holy Smoke 14: Down he goes with a lump on his scone like a rockmelon. A straight K.O.
1981		G. Young Slow Boats to China (1983) 43: As a boxer will study an opponent on film to decide whether to take it slow and tricky or go for a kayo in the first round.
1987		W.T. Vollmann You Bright and Risen Angels (1988) 325: This was K.O. on the O.K.
1993		T. Jones Pugilist at Rest 66: I lost on a K.O.
2000		F.X. Toole Rope Burns 16: Two wins by kayo.
2012		IOL News SA 20 Oct. 🌐 Chiefs suffer another early KO [...] a party atmosphere matched by a five-goal Telkom knockout thriller.

2. in fig. use, death or termination.

1918		letter in K.F. Cowing Dear Folks at Home (1919) 215: Everyone of their American boys are putting the K.O. on the Big, Brave, and Best German soldiers.
1939		V. Davis Gentlemen of the Broad Arrows 199: We all thought you had taken the K.O.
1940		E. O’Neill Iceman Cometh Act III: Some party last night, huh? Jees, what a funeral! It was jinxed from de start, but his tellin’ about his wife croakin’ put de K.O. on it.

3. attrib. use of sense 2.

1936		(ref. to 1920s) L. Duncan Over the Wall 71: This was almost a kayo punch good for a ten count.
1949		L. Glassop Lucky Palmer 125: A middleweight with a K.O. punch in either mitt.
c.1951		N. Algren ‘Watch Out for Daddy’ in Entrapment (2009) 136: [of knock-out drops] It’s time to go the kayo route, Baby.
2001		N. Barlay Hooky Gear 8: I done ju-jitsu an busted Duanes KO hand one time.