The name Mac in a Sac – technically Mac in a Sac 2 Packaway Jacket – still has a better name than jacket in a bag, but either way you get the picture. This simple concept, which is exactly what it sounds like a basic polyester shell that can block wind and water, fits in a small bag, a.k.a. the “sac.” The 100 percent waterproof yet breathable fabric makes it ideal for having some protection from the elements.