Computer Aided Design. Its what people use to design pretty much everything that exists in the real world, there are lots of subcategories for different uses. Architects use CAD to design buildings and bridges, mechanical designers use CAD to design household appliances and other objects, and electrical engineers use CAD to design circuit boards and silicon chips. In this case he is likely talking about using CAD software to design the gun that he 3d printed, this would have let him make shapes in 3d before sending them to a 3d printer.