A Waterloo combines bourbon, Dubonnet rouge, Grand Marnier (or Cointreau) and orange juice with a cherry for garnish. I got my recipe from Drinking French by David Lebovitz. He invented it as a response to a Napoleon cocktail that was disappointing. Get it? Napoleon? Waterloo?
My wife described the drink as "very interesting," noting a bitterness I would describe as woody. It undoubtedly comes from the bourbon.
The drink also brings to mind this song.