Any kind of deteriorated vehicle, usually the kind that mom and dad make you drive in high school. Popular shitwagons include the Chevette, Pinto, Yugo, Festiva, etc.
"You got a car, dude?"
"Yeah, but it's a real shitwagon."
A car in piss poor condition that not even a hobo would sleep in. Usually smells like the county landfill from it's previous owners' offspring spilling soda and crapping their pants in the backseats. Called a shitwagon because of it's similarity to what a farmer hauls fertilizer in.
"Dude my bitch gave me a Skinnamon Roll last night, it was so hot!"
"That's gross, we're not friends anymore; don't ever talk to me again."
"Whatever man!"