An "If-A-Cop" Story is one that you concoct when you're with a group of friends, doing something you're probably not supposed to, and you want everyone's stories to match up should you encounter some sort of law enforcement while out on your adventure/mission.
The best "If-A-Cop" stories are the simplest ones that answer these 3 questions:
~Where are you (all) going?
~Why are you going there?
~Where did you just come from?
The "If-A-Cop" story is ALWAYS discussed and shared with ALL members of the group BEFORE going anywhere and all members are also quizzed on the answers they will give should the "If-A-Cop" story actually be used because you've all been stopped by some sort of law enforcement personnel.
John, Mike and Mary are all walking to Tom's house to score some weed at 11 o'clock at night. Before they leave for Tom's:
John: Did we forget anything?
Mike: I don't think so...
Mary: Yeah we did! We need to come up with an "If-A-Cop" story!!! Ya know, in case the cops stop us...
John: Oh yeah! Good lookin' out!!!