Fling a turd that you created at an enemy in combat.

First you must create a turd (looting one in the sewers doesn't count). This can be done in any bathroom. Go to a toilet and press the A button to hop onto it. Then you'll have to mash the A button to fill the meter before the timer runs out. If you successfully fill the meter, the turd will come out of you and into the toilet. Press the A button again on the toilet to loot your turd. Now that you have a turd, in battle, go to your Items, and pick the turd, then pick an enemy to throw it at. This will Gross Out the enemy and will unlock the achievement immediately.