Green’s Dictionary of Slang

1. (UK prison) a rejection, esp. of parole.

2001

K. Sampson Outlaws (ms.) 31: It sounds like courtroom lingo [...] like a magistrate giving you a k.b.

2. any form of rejection.

1975		A. Bleasdale Scully 11: He’s giving his wife the old KB – you know, the Knock Back.
2001		I. Welsh Glue 81: So ye goat a k.b. then, Terry, eh?