Steal things with a total value of 30,000 Groschen.
To unlock this achievement you must steal goods with a total value of 30,000 Groschen. This will most likely unlock naturally throughout the course of the game. If not, you can steal expensive items from craft merchants and sell them to the millers. You may want to create a save before you do this so that you can revert back to it once the achievement is unlocked with untarnished reputation.