Garnish: Orange twist
Stir with ice. Strain into a cocktail glass.
Source: "The Savoy Cocktail Book" by Harry Craddock (1930)
From what I can figure out, a Diabolo is an odd-looking juggling device that almost looks like a yo-yo. Not quite sure what that has to do with this cocktail, but who am I to argue. I think the first appearance of this cocktail is in the 1930's "The Savoy Cocktail Book" by Harry Craddock, there he lists a recipe for six people, which I have scalled down here for a single drink.