Green’s Dictionary of Slang

(US black) a police officer, esp. a member of the drugs squad.

1950		Goldin et al. DAUL 119/1: Knock-man. A policeman; an informer; a complaining witness in criminal proceedings.
1992		R. Price Clockers 4: The white guy [...] looked too twitchy and scared to be a knocko.
1997		Simon & Burns Corner (1998) 16: Plainclothesmen. Knockers. Six police jump out of two unmarked Chevrolets.
2002		Simon & Burns ‘The Detail’ Wire ser. 1 ep. 2 [TV script] Western [Baltimore] knockos come around here, picking shit up off the ground.
2021		J. Fenton We Own This City 4: [T]he officers most likely to make up the plainclothes units known around town as ‘knockers’ or ‘jumpout boys,’ a reference to their aggressive tactics.