chopped liver
a) Because of its unusual taste and gray 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.

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


b) Literally, chopped liver is a traditional Jewish dish, cooked chicken livers chopped or ground and seasoned, sometimes used as a sandwich filling. "What am I, chopped liver?" means "Don't dismiss me as if I were insignificant." I recently heard an explanation of this saying. Unfortunately, I don't remember where. It went like this: Chopped liver is always served as a side dish, never as a main dish. It therefore makes a good metaphor for someone who's being treated as unimportant or dispensable.

http://www.phrases.org.uk/bulletin_board/16/messages/558.html