Describes the the common phenomenon regarding relationships between two members of the opposite sex. Proven true time after time, it has caused heartache of many men and women through the centuries. The abbreviation "Rule #6" comes from a list of advice posted online where this particular piece was 6th on said list.
Person one: "Hey, where's Max?"
Person two: "I don't know, we're not everywhere together all the time."
Person one: "It seems like it, you better be careful before Rule #6 strikes him down."