Eggnog's history is rooted in early medieval Britain, where a hot milk-based drink called "posset" was enjoyed. Posset often contained wine or ale, spices, and sometimes eggs. As the drink evolved over the centuries, eggs and milk became more prominent ingredients, particularly in the holiday season. By the 18th century, eggnog had made its way to the American colonies, where it became a popular festive drink. The availability of inexpensive rum from the Caribbean made it a suitable substitute for the wine or ale traditionally used in posset.