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You're a three decker sauerkraut and toadstool sandwich with arsenic sauce
In 1938 Ernst Hiemer wrote an anti-Semitic book that was published as a children's book. It's title, "Der Giftpilz" translated to, "The poisonous mushroom (or toadstool)". The German military ate sauerkraut due to its long shelf life and nutrient content. Dr. Seuss was a proponent of Jewish rights, and had experienced antisemitism personally. This line was most likely a jab artistically woven into this song about the Grinch striking back at anti-Semitic attitudes of the day.