Rich, custardy and full of calories, Eggnog has come to symbolise Christmas in contemporary times. It’s a seasonal indulgence, hitting store shelves during the chilly eggnog months of November and December. Despite its questionable nutritional profile though, eggnog has consistently been one of the most well-known beverages in the world. 
 
Eggnog, with roots tracing back to medieval times, evolved from the medieval posset—a blend of hot milk, alcohol, sugar, and spices. Posset, lauded for both pleasure and health benefits, even found its way into the prescription for King Charles I during a bout of cold in 1620.

British literature and domestic texts abound with mentions of various posset varieties, with Shakespeare tipping his hat to the drink in works like Macbeth and Hamlet. Despite being considered the precursor to the modern eggnog, many posset variations did not include eggs. 

Royalty, opting for cream or curds to thicken their posset, contrasted with commoners who turned to more affordable alternatives like eggs. A recipe from 1596's The Good Housewife's Jewel instructs one to gently heat a mixture of thick cream, sugar, ginger, and rose water, to concoct the perfect glass of posset. 

The precise moment when posset traversed the Atlantic remains elusive, but it is believed to have reached the Americas in the 18th century, likely accompanying British expansion in North America.The term "eggnog" appears to be of American origin, surfacing in the late 18th or early 19th century. It might have derived from "nog," an old English term for potent beer, or possibly from "noggin," a small wooden mug in which drinks were served in taverns. 

As the drink journeyed across the ocean, posset not only found its way to American shores but also reached Mexico and Puerto Rico, where each region adopted its unique version of the drink. While the core ingredients persisted, in Mexico, posset evolved into "rompope," featuring the addition of vanilla, and in Puerto Rico, coconut milk was introduced to create "coquito."

A significant transformation occurred when posset arrived in the Americas—the incorporation of alcohols like rum and whiskey. In England, these spirits were costly and scarce, leading to the use of sherry wine or the omission of alcohol altogether in posset. However, due to the proximity of the colonies to the Caribbean, rum and whiskey became readily available.

As eggnog gained popularity in America, it became closely linked with celebrations, especially during the holiday season. Even US Presidents like George Washington and Dwight D. Eisenhower had their own recipes for this festive drink. Surprisingly, eggnog played a central role in an infamous event known as the Eggnog Riot of 1826 at West Point. In response to Colonel Sylvanus Thayer's policy banning alcohol on campus, cadets, including future President of the Confederacy Jefferson Davis and his friend Robert E. Lee, organised a Christmas Eve party with smuggled liquor. The night escalated into chaos, resulting in fights, assaults on officers, and property destruction, leading to the expulsion of 19 cadets. 

Another version of this drink originated from Canadian soldiers during World War II, called Moose Milk. The recipes across the military branches were distinct in their own ways, but the ingredients remained the same – some form of liquor (typically whiskey, rum, and/or vodka), cream, egg yolks, and sugar. Canadian masses co-opted the drink to make it even more decadent, by adding Kahlua, ice cream, and coffee.