Green’s Dictionary of Slang

(a) (also slingback) a bribe, a gift.

1948		K.S. Prichard Golden Miles 74: ‘There’s some hungry bastards [...] makin’ big money on their ore, and never give the poor bugger boggin’ for them a sling back.’ The sling back might be ten bob on pay-day, or no more than a few pots of beer, but was always apprciated [AND].
1953		K. Tennant Joyful Condemned 232: Say I take twenty per cent of the cop for myself [...] all the rest goes in slings.
1967		‘Whisper All Aussie Dictionary’ in Kings Cross Whisper (Sydney) xl 4/5: sling: A bribe.
1975		Bulletin (Sydney) 26 Apr. 44/1: They’re willin’ all right. Real brutal. Different if you beat it for the sling, but just for the no-roast is unreal ....
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 9: They could afford to pay enormous slings to the police and politicians to keep the casinos operating.
1991		M.B. ‘Chopper’ Read Chopper From The Inside 80: Money for shady doctors, financing long-range campaigns, slings and backhanders.
2003		McGill Reed Dict. of N.Z. Sl. 192: sling A tip, bribe or bonus ANZ 1930s.
2006		B. Matthews Intractable [ebook] ‘We all know the jacks verbal crims. We all know the jacks cop a sling’.
2019		P. Temple ‘High Art’ in The Red Hand 25: ‘A bloke offered him a sling to pull the horse’.

(b) a share of criminal gains.

1950

S.J. Baker in Sun. Herald (Sydney) 8 June 9/4: ‘Corner’ or ‘sling’ means a share in a haul; and ‘not a droob’ means nothing at all.

(c) a tip.

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 11: The freight to Balmain had not left him with much shrapnel out of a deep-sea-diver. The driver did not get a sling.
1995		Syndeyy Morn. Herald 11 June 3/4: Jockeys have accepted rewards, or cash ‘slings’ for tips.
2003		see sense 2a.