a liquor made of spiced ale or wine, often with baked apples, and served in a large bowl, esp formerly, at Christmas and other festive occasions
revelry; carousing
The King doth wake tonight and … keeps wassail — Shakespeare
a toast to somebody's health made in former times
[Middle English wæs hæil from Old Norse ves heill be well, from ves imperative sing. of vera to be + heill healthy]