Green’s Dictionary of Slang

knockback n.

[SE knock back]

1. a rejection, a refusal.

[UK]Evesham Jrnl 21 May 7/1: He objected to the powers of the guardians being relegated to the officers. It was a knock-back .
[Aus]W.H. Downing Digger Dialects 31: knock-back — A refusal.
[Aus](con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: knock-back. A refusal.
[Aus]K. Tennant Battlers 143: The busker, always ready to save money, and realising from several ‘knock-backs’ that he had little hope of lodging in town, immediately pre-empted the vacant sleeping-place.
[NZ]F. Sargeson ‘That Summer’ in Coll. Stories (1965) 145: There was nothing doing. And I said to myself well, a knock-back from one of yous isn’t going to make me lose any sleep.
[Aus]D. Stivens Jimmy Brockett 30: Jimmy Brockett didn’t get many knockbacks when he set his mind on a girl.
[NZ]G. Slatter Pagan Game (1969) 164: Never had a knock-back in his life, that Tank Tarrant.
[UK]G.F. Newman Villain’s Tale 27: It was pointless making any moves [...] until he had enough of a firm to go out and do the blag. [...] When he got a couple of knockbacks he even considered reducing the number he would need, but finally decided four, including himself, was the absolute minimum.
[Aus]B. Humphries Traveller’s Tool 95: In my book no sheilah deserves to be called a lezzo unless she gives you the brush-off or the knock-back.
[Aus]M.B. ‘Chopper’ Read Chopper From The Inside 99: I offered $20,000 — and got a knock back.
[UK]J. Cameron Brown Bread in Wengen [ebook] Mrs Mellow never give any geezer the knock back.
[US]W. Shaw Westsiders 289: This has been a big knock-back for his confidence.
[Aus]B. Matthews Intractable [ebook] Probably the first knock-back she’s ever had in her life!’.
[Scot]T. Black Ringer [ebook] n.p.: Thinking about a lassie is the fastest road to a knockback. You never give them the space in your head.

2. (UK prison) the rejection of one’s application for parole.

[UK]J. Morton Lowspeak.
[UK](con. 1990s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 467: No lifer is ever granted first parole, and the minimum ‘knockback’, the period you must wait before applying again, is two years.

In phrases

get a knock back (v.)

(Aus.) to suffer a disappointment.

[Aus]Sport (Adelaide) 7 June 12/2: Jim P. [...] Thinks he’s on a clue. He would get a knock back if he knew the girls had him by the leg.
get the knockback (v.)

to be murdered.

[UK]J. Cameron Brown Bread in Wengen [ebook] On account of a fucking MP got the knockback they reckoned they wanted to explain it to the papers.