Doing laundry

Ooooh, scandalous~ This achievement requires you to sneak into a princess's room at night (or during the day depending on the princess' sleep schedule) to steal a pair of her panties. To do so you'll need to approach their castle at the appropriate time and slip by the guard on duty to enter. You can either use a party member skill to sneak by (moss princess and slime princess have the one you need) or use a sleep drug to knock them out. Sleep drugs and other assorted unpleasantness can be purchased from Trevor, the shady merchant who hangs around Faun's Tavern at night. Once inside the castle you'll need to avoid being spotted by the patrolling guards and reach the princess's chambers. There are certain items that make this easier but you can still do it by sheer sneakiness. Once inside the princess's room you just need to interact with the panties on her dresser and choose to steal them. Panty raid complete!