most people feel chopped liver is a digusting and undesirable food. it means in a way "what about me? am i undesired/ugly/unimportant/etc ?" of course the "am i ..." part varies with what you're talking about.
a common use for it would be when perhaps two girls are standing togther and all the guys are complementing one, the other might say, "What am I, chopped liver?"
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