Duck Soup, also called Ferret Soup and similar things, is a
high-calorie, high-protein concoction meant to be fed to old or sick
ferrets in order to fatten them up and help them regain their health.

To really get the weight back on a sick ferret, some people have
suggested giving him whipping cream.  It doesn't have much nutrition,
but it is full of calories and can help an underweight ferret gain
some back.

The following comes from Ann Davis:

ACME Ferret Company --- The Original  DUCK SOUP

For years, we have been trying to find a super formula to fatten up
sick ferrets, oldsters and ferrets with ulcers.  We have been looking
for something high in calories and protein, with added vitamins. After
trying just about everything on the market for pets, we had just about
given up, and were making do with some things that were not quite
perfect for the little guys, because everything made for cats that we
could find had a condensed milk base.

[If your ferret is really sick, you may have to work your way through]
all the steps, from full Sustacal to Duck Soup in caring for a sick
ferret.

We have heard of many miraculous recoveries attributed to Duck Soup.
It has helped old ferrets, ferrets with insulinoma, ferrets with hair
loss, and ferrets who are just plain too sick to eat.

DUCK SOUP

1 can Sustacal (8 oz., or about 230 ml; it comes in a larger size too) 
1 can water    (8 oz., or about 230 ml)  
2 scoops puppy or kitten weaning formula -- OPTIONAL
4 oz. (110 g? or ml?) dry kitten or ferret food, soaked in enough water to 
               cover and soften it completely

[Sustacal is meant for humans; look for it by baby formulas or in the
pharmacy section of your supermarket.  Debbie Riccio says you can also
use Ensure, Discover 2.0, or Just Born (puppy/kitten milk replacer).]

Mix thoroughly.  We always nuke it for them to the temperature of baby
formula.  We serve about 4 fluid ounces at a time twice a day for
maintenance; if your little guys eat too much and you feel they are
getting fat, you can increase the amount of water.  We have tried
increasing the amount of dry food, but if it gets too thick some of
them won't eat it. This formula also freezes well -- the Sustacal must
be used within 48 hours if left only in the fridge.