The world can be divided into two types of people: those who love eggnog and those who think it's gross. Okay, the world can be divided up in a lot more ways than that (western and eastern hemisphere or by favorite teenage mutant ninja turtle), but the point is that eggnog is quite divisive. While it's believed to have been first tied to the holiday season around the time of the American revolution, there's no real universal agreement on the origins of this milky, creamy, custard-y, egg-y holiday drink. So, grab a mug of the frothy stuff and come learn what the yolk is up with eggnog.
Culinary historians believe that a 13th century English drink called "posset" is the direct ancestor of the modern-day eggnog. Billed as "all the rage in the late Middle Ages" by the Glutton's Glossary, it was a concoction often made up of hot milk with warmed ale or sherry and mixed with sugar and spices. Monks sometimes added figs and eggs to it to symbolize good health and prosperity. As the years passed, it became more associated with the wealthy and actually made a passing appearance in Shakespeare's Macbeth as the drink Lady Macbeth poisoned the guards outside of King Duncan's quarters with so they would not hear her murder him. The drink more or less fell out favor by the 19th century, however, it's in midst of somewhat of a revival in the British brunching crowd.
The word "nog" may also stem from Shakespeare's era too. According to the Online Etymology Dictionary, the word dates back to the late 17th century and refers to a very specific type of strong beer that was brewed in Norfolk, England. It seems possible that "nog" was simply a local variation of "posset." Although one other theory about "nog's" origin is that it alludes to the type of cup it was served in, an insulating small wooden mug good at keeping the drink warm.