It refers to a certain design of a car that has an unbroken curved line from the top of the roof to the rear bumper. The rear window slope follows the unbroken roofline and often remains at less than a 45-degree angle.
A car that has an unbroken curved line from the top of the roof to the rear bumper as opposed to a drop in the line for a near-vertical rear window. In a fastback design the rear window slope follows the unbroken roof line and is often at less than a 45 degree angle.
A. Car with sloping back glass. B. Where Front Row Joe goes, and how he gets there, as soon as the green flag flys.