“Duck sauce” is often served at American Chinese restaurants, accompanying noodles, egg rolls, chicken and spare ribs. “Duck sauce” was sold in bottles and packages to be sauce for Peking duck, but the proper sauce for that dish is hoisin sauce. “Duck sauce” is also called “plum sauce” and can contain plums, apricots and peaches in addition to sugar, vinegar, ginger and other spices.
 
“Duck sauce” is cited in print by at least 1938.
 
   
Wikipedia: Duck sauce
Duck sauce is a translucent sweet and sour orange condiment used in some Chinese-American restaurants. It may be used as a dip for deep-fried dishes such as duck, chicken, fish, spring rolls, egg rolls, or with rice or noodles. It may be made of apricots, plums, or peaches added to sugar, vinegar, ginger and chilis. It is rarely, if ever, used in traditional Chinese cooking. Due to fruit content it may be called “plum sauce”.
   
About.com: Chinese Food
Duck Sauce - Glossary of Chinese Ingredients and Cooking Terms
By Rhonda Parkinson, About.com
Definition: Duck sauce is another name for Plum Sauce. The name “duck sauce” comes from the fact that Western Chinese restaurants began serving it with the pancakes that go with Peking Duck, under the mistaken impression that this was an authentic practice. (In China, Peking Duck is traditionally served with hoisin sauce). But whatever name it goes by, plum sauce makes an excellent dipping sauce for appetizers, such as Egg Rolls and Spring Rolls.
   
wiseGEEK
What is Duck Sauce?
It’s hard to think about Chinese food without also thinking of duck sauce. While this tasty sweet and sour sauce has a long association as the perfect compliment to Chinese cuisine, duck sauce can be used as a glaze and an ingredient in recipes as well as being an ideal dipping sauce. Here is some information about duck sauce and some of the ways that this Asian sauce can be used.
 
The basic ingredients duck sauce can be found in just about any supermarket. All you need to make duck sauce are some dried plums, dried apricots, vinegar, sugar and a few spices. Because of the content of the dried fruit, duck sauce is sometimes referred to as plum sauce or apricot sauce. In fact, when the sauce is used as an ingredient in some recipe, it will often be listed as a plum or apricot sauce instead of duck sauce.
 
What makes duck sauce so tasty with many Chinese dishes is the sweet and sour flavor combination. Many Chinese dishes utilize portions of chicken or fish that are deep-fried. Using a fork or chop sticks, a bite sized portion of the fish or chicken is dunked in the duck sauce before eating the portion. The result is a sweet and sour taste that is combined with the satisfying crunch of the fried batter. Along with the fried meats, duck sauce is also great for dipping an egg roll or even mixing in fried rice.
   
ChineseFoodDiy.com
Duck Sauce
Yield: 8 cups
Ingredients:
1 pound plums, halved and pitted
1 pound apricots, halved and pitted
1-1/4 cups cider vinegar
3/4 cup water
1 cup cider vinegar
1 cup firmly packed brown sugar
1 cup white sugar
1/2 cup lemon juice
1/4 cup chopped ginger
1 small onion, sliced
1 Serrano pepper (or more), seeded and chopped
2 small garlic cloves, sliced
4 teaspoons salt
1 tablespoon mustard seed, toasted
1 cinnamon stick (...)
     
Google Books
Cook at Home in Chinese
By Henry Low
New York, NY: Macmillan
1938
Pg. 5:
SAUCES
Duck Sauce* (so moue jeung)
A kind of chutney good with any kind of duck
 
21 January 1951, New York (NY) Times, “Chinese Pork Dishes,” pg. 158:
Serve hot and crisp as an appetizer with mustard and duck sauce (a bottled, sweet sauce from China, also sold in Chinatown).
       
9 May 1957, Blytheville (AR) Courier News, “Pineapple, Soy Sauce Add to Sparerib,” pg. 14, col. 3:
Serve with Chinese style hot mustard and Chinese-style sweet fruit sauce (sometimes called Duck Sauce and put up in bottles).
 
Google Books
Linda
By Eleanor B. Wasserman
Published by E.B. Wasserman
1958
Pg. 34:
I liked Chinese food, too. Egg rolls are good with duck sauce, made out of peach jam and garlic and red pepper and french fried mano clams are wonderful.
     
Google Books
Chinatown, U.S.A.
By Calvin B. T. Lee
Published by Doubleday
1965
Pg. 80:
There will be jars and cans of shuen moy jeung (commonly called duck sauce in Chinese restaurants) into which spare ribs, roast duck and egg rolls are ...
   
22 July 1965, New York (NY) Times, “Chinese Woks” by Craig Claiborne, pg. 20:
Serve with duck sauce or plum sauce (available in bottles in Chinese grocery stores) as a dip.
   
Google Books
Sunset Oriental Cook Book
Edited by M.R. Piper
Published by Lane Books
1970
Pg. 12:
Canned Chinese plum sauce, soh mui jeong (C), often is a spicy accompaniment to roast duck and sometimes labelled “duck sauce.”
   
Google Books
The Everything Chinese Cookbook:
From Wonton Soup to Sweet and Sour Chicken—300 Succulent Recipes from the Far East
By Rhonda Lauret Parkinson
Published by Everything Books
2003
Pg. 29:
What about Duck Sauce?
Not sure whether to serve duck sauce or plum sauce with your mandarin pancakes? Actually, both terms refer to the same sweet and spicy sauce. Plum sauce was nicknamed “duck sauce” after Western Chinese restaurants began serving it with Peking Duck, under the mistaken impression that this was an authentic practice. In reality, Peking Duck is traditionally served with hoisin sauce.