Comes from the infamous "Car lot lingo". A "Jack" is a customer that strolls into a car lot usually holding a note pad or a portfolio of some sort with quotes from other dealerships, specs of vehicles etc. Sometimes Jacks may even show up with a measuring tape of some sort. The reason they are referred to Jacks is because they "Jack off" the car salesman to get information out of the salesman with out any intention of buying a car from them. In other words a Jack will shop you and research you and buy from somewhere else thus the term "Jack" for "Jacking you off" Only seasoned and experienced Car Salesman can spot a Jack at first glance, its usually the Green Peas or the inexperienced salesman that gets Jacked royaly with out even knowing it
John: "Ive got these two walkers in the front line!"
Peter: "Wow!, good call John!! hope you can sell them a car!!"