By what term was eggnog known during the early years in medieval Britain?
The Oxford English Dictionary related that posset was a favored drink in medieval Britain. Posset consisted of hot milk curdled with wine, ale or similar additive, then sweetened and spiced. Later on, monks added whipped eggs to the mix. Some monks later added figs into the mix. Toasts to prosperity and good health favored posset. George Washington is famously known to have served guests an eggnog-like drink that included cherry, rye whiskey and rum. Source: PBS.org
A posset (also spelled poshote, poshotte) was a hot British drink made of milk curdled with wine or ale, often spiced, which was popular and used as a cold and flu remedy. The word is mainly used nowadays for a related dessert similar to syllabub.