Term used by the character Elias in the Kevin Smith movie Clerks II. It is a "troll" placed by one's parents in a girl's mouth at a young age in order to prevent open-mouth kissing before the age of 21.
See also: pillow pants
Randal: Have you and Myra even kissed yet?
Elias: We would have already if it wasn't for Listerfiend..
Randal: Listerfiend is her mouth troll, isn't it..