A code of secrecy invoked by promising to never tell another person the story you are about to be told.
In a conversation the Mooney Rule would be used as follows:
Person 1: Dude, I have something important to tell you, Mooney Rule on this.
Person 2: Ok, I swear never tell anyone.
Person one then proceeds to tell person two the important story knowing that the secret will stay private between the two due to the Mooney Rule being invoked.