A small four-cylinder vehicle (i.e. Honda Civic) usually painted yellow, pink, or purple and driven by a 5'5" jackass. The wagon usually has a large wing on the back, white rims, and an annoying loud muffler.
Very similar to a butt wagon, though a bit more oriented toward people who deserve a heavier term than butt wagon. A douche wagon can also safely be called a buttsnoipe, an asshaberdasher, a bunghole, etc.