This achievement is called "Too Far" and is for farting on the corpse of an aborted Nazi Zombie Fetus.
When you're on the quest in the clinic to help the girls, you will eventually come across these creepy German-speaking things. Once you finish fighting your first group of them, simply cast your fart ability on their corpses and you should hear them swear in German if you successfully farted on them. Voila! Achievement unlocked.
If you do happen to miss this on that particular quest, if you return to the Unplanned Parenthood building soon after, the giant fetus should still be there for you obtain the achievement.
Also, feel free to keep farting on them for comedic effect