In a literal sense, a person who masturbates, but in a more commonly used way in the United Kingdom, an idiotic person who you hate. If you call someone a wanker, you are most likely referring to the more informal use of the word and calling them a wanker, but the
irony of this insult is that it's probably true in a literal way (for about 95% of
people), but you hardly ever
mean it that way.