Green’s Dictionary of Slang

knocked off adj.¹

stolen.

1930		(con. 1910s) D. Mackenzie Hell’s Kitchen 117: ‘Knocked off’ is also applied to anything ‘pinched’ (stolen).
1935		G. Ingram Cockney Cavalcade 125: I told you that they’d been ‘knocked-off’ and I got ’em for half-price.
c.1943		‘Bill O. Lading’ You Chirped a Chinful!! n.p.: Knocked Off: Stolen.
1960		H. Pinter Caretaker Act I: I had a tin, only ... only a while ago. But it was knocked off.
1972		P. Fordham Inside the Und. 162: Watching TV in a knocked-off silk dressing-gown.
1982		J. Sullivan ‘The Long Legs of the Law’ Only Fools and Horses [TV script] Knocked off are they?
2000		Guardian Guide 8–14 Jan. 52: Burnside has fire arms and bundles of high-grade knocked-off cocaine in his boot.