When someone says "six up", it means the police are nearby, so whatever you are doing that is illegal you'd better hide it.
This phrase is most commonly heard in parking lots at Phish concerts where many people openly sell or use drugs, or illegally vend food or beer. "Six up!" is used to give people a heads up when the cops are coming around or when there is an undercover cop trying to bust people.
Military expression for "watch your back"
Commonly shouted by nudists on Wreck Beach, Vancouver, British Columbia, Canada when the cops are on the beach. Known mostly to locals as a warning to stop doing anything illegal.