scofflaw |ˈskôfˌlô; ˈskäf-|noun informala person who flouts the law, esp. by failing to comply with a law that is difficult to enforce effectively.
I learned this word watching a Ken Burns documentary about the Prohibition. Apparently, the Boston Herald held a competition for readers to come up with a word to describe those who broke the new law. Two unrelated entrants submitted "scofflaw" and it was chosen as the winner.