Green’s Dictionary of Slang

1. (also whack off) to consume, to eat or drink.

1965		H.S. Thompson letter 3 May in Proud Highway (1997) 515: My advice to you is to get out of hospital and whack off a bottle of John Powers Irish.
1986		T. Philbin Under Cover 185: Edmunton whacked down his drink.
1998		P. McCabe Breakfast on Pluto 85: Quaffin’ the old vino and whackin’ em all dahn, them great big steaks!

2. to inhale.

2000

Indep. Rev. 4 Feb. 12: I know people who went and bought glue [...] They got a bag, went up the fields and started whacking loads of the stuff down.