Earn the Vulgar Thrust expression, or you can buy it (The Perv's Handbook - Vulgar Thrust from the Bowerstone Market book store) Then, go to a civilised place, (Fairfax Gardens or Bowerstone Market). In the menu, Clothing>Outfits>No Clothes, and then run around and do the Vulgar Thrust expression to people. When someone is offended, they'll report you and you'll get a 10 gold fine for Public Indecency and the achievement.