1. a contemptable person; jerk.
2. a male masturbator.

Contradictory to belief, a wanker is not someone who masturbates.
A wanker is someone who is an idiot

n. English Slang - A fellow road user. see also asshat cunt fucktard

The other contributors are correct in the literal meaning of the word as "one who masturbates", however in Australia, it is used more like a slightly more vulgar synomym for words like fool or idiot

Someone who beats off a lot or too much.

A man that enjoys masturbation also a common insult.

Wanker: An arrogant or pretentious person. Sometimes this word is used instead of resorting to 'Cunt' or 'Bellend'.