I use this phrase from time to time, but I only recently learned what it means. According to Jewish culinary tradition, chopped liver was served as a side dish and not as a main dish, so the meaning of the phrase was obvious to everyone who ate. But these days chopped liver is a far cry from your normal side dish, particularly in America, and particularly in the non-urban mid-west. I would actually have to drive three hours north to the Chicago suburbs, or two hours west, to Indianapolis, to even order chopped liver as a side dish at a restaurant (but someone else would have to eat it).
It turns out that I have been using the phrase incorrectly anyway. Most of the time I feel like I should be saying, "I'm not even chopped liver," because more and more it feels like what I do doesn't even classify as a side dish in the dinner party of life. But that doesn't sound as good. I'm hoping that the feeling will pass, like the indigestion and heartburn that would eventually pass after eating real chopped liver.
It is possible to make mock chopped liver out of cooked lentils, sautéed onions, ground-up walnuts, a bit of soy sauce, a bit of miso, and a bit of broth. There's another recipe I found that uses sautéed, chopped mushrooms rather than lentils.