Green’s Dictionary of Slang

whacker n.4

also wacker
[whack off v.]

1. (US) a masturbator, lit. and fig.

[US]L. Bruce How to Talk Dirty 6: He knew he had a pervert living under his roof [...] a whacker!
[Aus]A. Weller Day of the Dog 48: Ya want to look out for those wackers.
[US]H. Max Gay (S)language.
[Aus]T. Winton Lockie Leonard, Legend (1998) 39: ‘Garry Pooley is a dweeb and a drongo.’ ‘A wacker, actually.’.
[NZ]McGill Reed Dict. of N.Z. Sl. 225: wacker/whacker 1. Male masturbator. ANZ. 2. Stupid and/or nerdy person. ANZ.

2. as (affectionate/teasing) term of address.

[UK]K. Amis letter 12 July in Leader (2000) 211: I hope too that I get a bloody job before long, or I s’ll be on the breadline, wacker.
[UK]K. Amis letter 5 May in Leader (2000) 319: Now’s your chance, whacker.
[UK]G.W. Target Teachers (1962) 25: Sorry, wacker.
[NZ]McGill Dict. of Kiwi Sl. 122/2: whacker wanker or masturbator, used as term of abuse, affection or affectionate abuse.