Spirit + eggs + milk + sugar served cold or with hot water added
Today, we all know the egg nog as a traditional drink of Christmastime. Unfortunately, our memories of it are mostly colored by the horrid mixture that comes out of a cardboard carton. Traditionally it would be made with some type of spirit (Rum and Brandy most preferred), eggs, milk, and sugar. If served cold it would be mixed with ice, if served hot it would instead have hot water added to it and served in a mug.
For an example, see: