Green’s Dictionary of Slang

sling n.1

[SE sling, to throw (back)]

1. [late 18C] a draught, a ‘pull’ at a bottle or glass.

2. [1940s+] (Aus.) in monetary senses.

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

(b) a share of criminal gains.

(c) a tip.