Tractor Girl Soup is from the movie The Crazies. A scene that involves a tractor and a guy and girl which ends up making them look like soup. (guts and blood everywhere)
It's just a funny, very random answer you give to anyone if the question they asked has the qualities of the above description.
Girl: Err...Tractor Girl Soup...
Person 1: Hey! Welcome to McDonalds! May I take your order?
Person 2: Umm..Tractor Girl Soup, Tractor Girl Soup..uh..Tractor..Tractor..Soup..I'll have the Big Mac.