When in bathrooms, you will be able to go for a number 2 on the toilet, and if successful, you will get a lovely piece of poo for you to grab.
When in battle, you'll have the option to use consumables (the potions icon). Pick this and scroll to the bottom to use the poo. Then chose whoever you like to throw the poo at so they'll be "grossed out". The achievement will unlock after you've done this.