Green’s Dictionary of Slang

knock-over n.

[knock over v.]

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

[Aus]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.

[US]J.J. Finerty Criminalese.
[US]Howsley Argot: Dict. of Und. Sl.

3. (US police) an armed robbery.

[US]D. Hammett ‘The Big Knockover’ Story Omnibus (1966) 293: He started peddling these pamphlets the day before the knockover.
[US]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.
[US]‘Hal Ellson’ ‘Cool Cat’ in Tell Them Nothing (1956) 75: You told us it was an easy knockover.

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

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

5. (US) an easy task.

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