mass noun archaic
1 Spiced ale or mulled wine drunk during celebrations for Twelfth Night and Christmas Eve.
1.1 Lively and noisy festivities involving the drinking of plentiful amounts of alcohol; revelry.
[no object] archaic
Drink plentiful amounts of alcohol and enjoy oneself with others in a noisy, lively way.
Middle English wæs hæil ‘be in (good) health!’: from Old Norse ves heill (compare with hail). The drinking formula wassail (and the reply drinkhail ‘drink good health’) were probably introduced by Danish-speaking inhabitants of England, and then spread, so that by the 12th century the usage was considered by the Normans to be characteristic of Englishmen.