The Australian slang version of the English "wanker".
Slight difference in that wackers tend to be more boisterous than wankers. Wankers kind of like to do things sneakily, and probably harbour a fear that people are calling them wankers behind their backs.
Wackers, on the other hand, are oblivious to what people say about them and seem to even believe their own bullsh!t.
There are more Australian wackers than wankers, but believe me - we do have both kinds.
Did you hear Bush talking to those sailors on that battleship? Honestly, the wacker thought they were laughing WITH him.