Eggnog is a traditional 'American' drink but it started life in the UK as a kind of 'posset' (hot milk mixed with wine or ale and spices). There are a very few old mentions of eggs being mixed in as well.
During the middle ages, possets were used to help treat colds and the flu. (Now the word posset is normally used for a cold set milk pudding which was developed from the drink in the 16th century.)
'Posher' possets were also drunk by the upper classes in the UK, where it was made with Sherry or Brandy rather than beer or ale.
There are some different theories about how the 'nog' in eggnog came about. One theory says that 'nog' was the name given to strong beer in East Anglia and when eggs were added to it, it became eggnog. Another theory says the nog comes from the word 'noggin' a middle English word for a small and carved wooden mug which was used to serve alcohol (and noggin is still a song term for your head!). And yet another say the nog comes from nugg or nugged ale, a Scottish term meaning ale which had been warmed by putting a poker from the fire in it!
In the mid 1700s the drink travelled over to the USA, where sometimes rum was used in it rather than ale. And from that there's another naming theory, as rum was known as grog, so eggnog could have started out as egg-n-grog.
But I think the eggs and strong ale or eggs in the noggin cups makes more sense...
The first written use of eggnog is from 1775 when Jonathan Boucher, a clergyman and philologist (someone who studies old texts) from Maryland, wrote a comic poem about the various drink he had during the day! (But his poem wasn't publish until after his death 30 years later.)
"Fog-drams i' th' morn, or (better still) egg-nogg,
At night hot-suppings, and at mid-day, grogg,
My palate can regale..."
In March 1788 a newspaper in New Jersey reported that:
"A young man with a cormerant appetite, voraciously devoured, last week, at Connecticut farms, thirty raw eggs, a glass of egg nog, and another of brandy sling."
The earliest connection of Christmas and eggnog is from the Virginia Chronicle in 1793:
"On last Christmas Eve several gentlemen met at Northampton court-house, and spent the evening in mirth and festivity, when EGG-NOG was the principal Liquor used by the company. After they had indulged pretty freely in this beverage, a gentleman in company offered a bet that not one of the party could write four verses, extempore, which should be rhyme and sense..."
An early eggnog recipe comes from 1799 when the book 'Travels Through the States of North America and the Provinces of Upper and Lower Canada' (volume 2) described how and inn in Baltimore made eggnog:
"The American travellers, before they pursued their journey, took a hearty draught each, according to custom, of egg-nog, a mixture composed of new milk, eggs, rum, and sugar, beat up together;..."
Also in the 1790s, George Washington was the first US President to serve eggnog. His recipe contained rum, whisky and sherry!
The 'Tom and Jerry' is an eggnog cocktail made with rum and brandy. It was invented in 1920 by the British journalist Pierce Egan. It's thought that the Tom and Jerry cartoon might be named after this cocktail!
In the 1892, 'The Medical Brief' journal suggested that eggnog could be used as a treatment for the flu - so going back to how medieval possets were used!
The eggnog you can now get in cartons, isn't normally alcoholic.