What does the slang expression duck soup mean?

"Duck Soup is a 1933 Marx Brothers' anarchic comedy film written by Bert Kalmar, Harry Ruby, Arthur Sheekman, and Nat Perrin and directed by Leo McCarey. It starred what were then billed as the "Four Marx Brothers" (Groucho, Harpo, Chico, and Zeppo) and also featured Margaret Dumont, Raquel Torres, and Louis Calhern. It was the last Marx Brothers film to feature Zeppo.

McCarey came up with the title for the film. This was in keeping with the "animal" theme of the brothers' previous three films, Animal Crackers, Monkey Business and Horse Feathers. "Duck soup" is a slang phrase meaning something easy to do. When Groucho was asked for an explanation, he quipped, "Take two turkeys, one goose, four cabbages, but no duck, and mix them together. After one taste, you'll duck soup for the rest of your life."

http://en.wikipedia.org/wiki/Duck_Soup

Duck is a very fatty meat. Therefore a rich soup stock is easy to concoct. The flavor is strong enough to enhance even insipid vegetables.