See also: Listerfiend
As an Adjective: A person who on more than one occasion has prevented a sexual act from occurring.
As a Verb: The act of preventing a sexual act from occurring.
The term itself comes from the film, "Clerks 2" in which the pillow pants is a troll living in a girls vagina that bites off a guy's penis if he tries to have sex with her before her 21st birthday.
Verb: I totally pillow pantsed my brother yesterday. I walked in just as he was taking her shirt off!