Green’s Dictionary of Slang

1. (Aus.) a shock, a disappointment.

1904

Sun. Times (Perth) 6 Mar. 8/3: That it should have been Ivy Lawson charged with such a dirty low-down trick [...] is a bit of a knock-over.

2. (US Und.) a police raid.

1926		J.J. Finerty Criminalese.
1939		Howsley Argot: Dict. of Und. Sl.
1951		C. Stoker Thicker ’n Thieves 325: The following night, I was ready for the knockover. [...] I would raid the joint [i.e. an unlicensed club].

3. (US police) an armed robbery.

1927		D. Hammett ‘The Big Knockover’ Story Omnibus (1966) 293: He started peddling these pamphlets the day before the knockover.
1940		W.R. Burnett High Sierra in Four Novels (1984) 290: This is a soft spot. They’ve never had a knock-over in Tropico Springs.
1956		‘Hal Ellson’ ‘Cool Cat’ in Tell Them Nothing (1956) 75: You told us it was an easy knockover.
1994		G. Pelecanos Shoedog 64: A standard knockover [...] you go in with a hard look and a drawn gun, maybe rap the barrel to someone’s head, let them feel the weight, and then you book.

4. (Aus.) a substantial, if surprising, success.

1955

N. Pulliam I Travelled a Lonely Land (1957) 235/1: knock-over – an outstanding success.

5. (US) an easy task.

1977

Schrader Blue Collar 50: ‘Is it a knockover or not?’ ‘It’s a knockover all right. Baby food.’ [HDAS].