Green’s Dictionary of Slang

1. killed, murdered.

1928		(con. WWI) Wharton ‘The Squad in Action’ in Mason Fighting American (1945) 511: ‘Marzaluk’s knocked off,’ and he points to the body.
1936		J. Curtis Gilt Kid 34: What the hell had happened to all the blokes he used to know who had used Isabella’s? All got knocked off or something.
1938		H.M. Anderson Strip Tease 33: ‘Since her guy got knocked off she hasn’t been seen around’.
1948		N. Mailer Naked and Dead 120: They’re figuring out what to do with the ten thousand insurance on me when I get knocked off.
1955		‘Ed Lacy’ Best that Ever Did It (1957) 12: Jeez, a cop was knocked off!
1982		G.V. Higgins Patriot Game (1985) 163: He was cooperating in some enterprise that seemed pretty likely to get my no-good brother knocked off.

2. beaten; defeated.

1972

C. Bukowski Erections, Ejaculations etc. 441: Champions get the razzberry; the crowd aches to see them get knocked off.