The story
It is said to taste like the soft drink Dr Pepper, though not using it as an ingredient.
Etymology from Wiktionary, CC BY-SA 4.0.
Flaming Doctor Pepper
Today it means: A cocktail made by filling a shot glass with three parts amaretto and one part high-proof liquor — layered, not mixed — then setting it on fire and dropping it into a glass of beer, which extinguishes the flames. Full definition →