This is a phrase from Kurt Vonnegut's masterpiece, Slaughterhouse-5, The novel uses certain phrases repetitively, such as "so it goes"—which, used whenever death or dying is mentioned (be it that of a man, an animal, or the bubbles in champagne), serves to downplay mortality, making it routine and even humorous—and "mustard gas and roses", to denote the horrible odor of a rotting corpse or a drunk's breath.