Egg yolks beaten with sugar, then mixed with very hot cream and milk and seasoned with cinnamon or nutmeg. With all due respect to Sheldon in Big Bang Theory, eggnog is THE traditional Christmas drink. A rich, creamy, comforting cocktail, ideal for warming the mind and body, or to combat a cold or fatigue.
The alcohol version was all the rage among English aristocrats before being imported to colonial America in the eighteenth century, where rum (or grog in slang) replaced liqueurs such as the overly expensive sherry or brandy. The term "eggnog" is a contraction of "egg and grog". Today Americans mostly prefer the cold alcohol-free version.