Chopped liver became a popular dish in the American Jewish cuisine in New York City by at least 1910. The liver is often made with chopped egg, salt, pepper and moistened with chicken fat; fried onions are often mixed in as well. The dish is regarded as an appetizer, often eaten with bread or crackers.
   
Chopped liver became so common that the phrase “What am I? Chopped liver?” became widely used by the 1940s and 1950s.
     
   
Wikipedia: Chopped liver
Chopped liver is a spread popular in Jewish cuisine.
 
It is often made by sautéeing or broiling liver and onions in schmaltz (i.e., rendered animal fat); adding hard-boiled eggs, salt and pepper to the sautéed liver and onions, and grinding that mixture. However other methods and materials exist and so the exact process and ingredients may vary from chef to chef.
 
Chopped liver is a common menu item in Kosher delicatessens in the U.S. and Canada. Chopped liver is often served with rye bread as sandwiches.
 
The liver used is generally calves’ liver or chicken liver. Shortening or oil is often substituted for the schmaltz.
 
Variations
Because of the liver, chopped liver is high in protein but also high in fat and cholesterol. Thus, low fat, mock, and vegetarian versions of chopped liver exist that are frequently made of a combination or base of peas, string beans, eggplant, or mushrooms.
 
Chopped liver in popular culture
Because of its unusual taste and appearance, it is an acquired taste and not a favorite or comfort food with everyone at the dinner table. This has given rise to the popular Jewish-American expression “What am I, chopped liver?”, signifying frustration or anger at being ignored on a social level.
 
An alternate explanation for the etymology of the “What am I, chopped liver?” expression is that chopped liver was traditionally served as a side dish rather than a main course. The phrase, therefore may have originally meant to express a feeling of being overlooked, as a “side dish.”
 
19 December 1897, Sunday News Tribune (Duluth, MN), “Uncle Sam’s Many Dinners,” section 3, pg. 19:
The Russian Christmas dinner consists of a roasted pig, which is raised for that day as we raise the goose and the turkey. It is stuffed with kasche (boiled buckwheat) mixed with chopped liver, heart and giblets.
 
28 November 1914, Fort Wayne (IN) Sentinel, “Joe the Waiter,” pg. 27?, col. 1:
I AM sorry, but we are all out of pickled fish. You can have some chopped liver instead, and you can have some fine cabbage soup after that.
 
Google Books
An American in the Making:
The Life Story of an Immigrant
by M. E. Ravage
New York, NY: Harper & Brothers
1917
Pg. 226:
At home we began the big meal of the day with radish or ripe olives or chopped liver or fish; then we had meat of one kind of another; then some vegetables cooked sweet or sour-sweet, and wound up with soup.
 
Pittsburgh Jewish Newspaper Project
30 November 1923, The Jewish Criterion, pg. 41, cols. 2-3:
Entree of Chopped Liver
One onion, one tablespoon chicken fat, any amount of chicken livers (or calves liver), three hard boiled eggs. Bake liver in oven and put with onion and eggs through meat grinder. Season with salt and pepper and moisten with chicken fat.
 
8 September 1928, Dallas (TX) Morning News, “Sidewalks of New York” by Loia Cheaney, pg. 5:
And, although, if you remember your Old Testament, you will realize these recipes have been handed down quite a while through the ages, nevertheless you feel somewhat like a pioneer explaining them. Chopped liver, chopped so fine it is almost a paste, is probably the best appetizer one can eat.
 
8 April 1929, Oshkosh (WI) Daily Northwestern, “Enjoys Jewish Foods,” pg. 1, col. 3:
Mount Clemens, Mich.—(UP)—Henry Ford enjoyed his first taste of Jewish foods here Sunday. Ford was served gefulte fish, kosher chicken and all the side dishes. The auto manufacturer was eager to try chopped liver and egg but his order could not be filled.
 
20 September 1934, Palestine Post, pg. 6:
Chopped Liver Shamrocks
Chop your liver as usual and stuff into a pepper shell very tightly. Then slice it across and it will look like a shamrock. Serve on Slice tomatoes for colour effect.
 
22 August 1937, New York (NY) Times,  “Gay, Incredible Coney; Portrait of a Multitude” by Victor H. Bernstein, pg. SM11:
“Papa, hold for a minute the pickles; the fat man kicked all over the chopped liver some sand.”
   
The WPA Guide to New York City
New York, NY: Random House
1939
New York, NY: Pantheon Books
1982
Pg. 26:
JEWISH. Gefulte fish (spiced fish cakes, served cold); sour cream mixed with fruit, vegetables, or pot cheese; chopped liver, usually mixed with fried onions and chicken fat; kugel (potato or noodle pudding); noodles and cottage cheese.
   
26 January 1939, Palestine Post, pg. 4:
A mock chopped liver can be made of kidneys fried with onions and minced up with hard boiled eggs.
 
January 1951, Brooklyn (NY) Yellow Pages, pg. 731, col. 3:
LITTLE ORIENTAL RESTAURANT INC
“FAMOUS FOR CHOPPED LIVERS”
STEAKS . CHOPS
1546 PitkinAv…DTms 2-8731