Green’s Dictionary of Slang

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

1965		L. Bruce How to Talk Dirty 6: He knew he had a pervert living under his roof [...] a whacker!
1981		A. Weller Day of the Dog 48: Ya want to look out for those wackers.
1988		H. Max Gay (S)language.
1997		T. Winton Lockie Leonard, Legend (1998) 39: ‘Garry Pooley is a dweeb and a drongo.’ ‘A wacker, actually.’.
2003		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.

1949		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.
1953		K. Amis letter 5 May in Leader (2000) 319: Now’s your chance, whacker.
1960		G.W. Target Teachers (1962) 25: Sorry, wacker.
1988		McGill Dict. of Kiwi Sl. 122/2: whacker wanker or masturbator, used as term of abuse, affection or affectionate abuse.