This season, you might hear (or sing yourself) the Christmas carol that begins, "Here we come a-wassailing / among the leaves so green." As is holiday tradition, you will wonder: what in the world is "a-wassailing" and why is it happening among green leaves in the dead of winter? And what about that refrain?
Love and joy come to you
And to you good wassail too...
Wassailing is an old custom that goes back to the 1300s. The verb wassail derives from the noun wassail, which dates to the 1200s and was first used to refer to an Old English custom of hospitality. In medieval England, a courteous host would offer a cup to a guest and toast them with wæs hæil, or "be in good health." The guest would accept the cup and respond with drinc hæil, "drink in good health." This custom, and the word wassail, were both adopted by the English from the Nordic Vikings: waes haeil is from the Old Norse ves heill, "be in good health."
In short order, wassail was also applied to the party at which the wassail was offered, as well as the actual drink passed around. By the 1500s, it was used to refer specifically to a drink served at Christmastime (and especially on Twelfth Night):
ITEM, as for the voide on twelfth day at night, the King and Queene ought to take it in the halle; and as for the wassell, the steward nad treasurer shall come for it with their staves in their hands... and when the steward cometh in at the halle door with the wassell, he must crie three tymes, Wassell, wassell, wassell...
—"Articles Ordained by King Henry VII for the Regulation of His Household, 31 Dec. 1494" in A Collection of Ordinances and Regulations Made for the Government of the Royal Household, 1790
Wassailing, then, originally referred to "keeping wassail": gathering together and drinking each other's health. As the drink became associated with Christmas, wassailing itself changed. The meaning of the verb wassail as it shows up in the carol refers to going around, caroling, and wishing those you visit good health, and excellent wassail, or holiday parties.
(The carol is much newer than wassailing: it was written in the mid-1800s.)