Wassail
1. A salutation used when presenting a cup of wine to a guest, or drinking the health of a person (to which the correct reply is “drink-hail”) and thus by extension any general salutation.
2. The liquor in which healths were drunk and particularly the spiced ale used in Twelfth Night and Christmas Eve celebrations.
  
3. A custom observed on Twelfth Night and New Year’s Eve of drinking healths from the wassail-bowl and hence a carousal or riotous festivity.
4. A carol or drinking song sung by wassailers.
5. To drink to the health of fruit-trees, cattle etc. in wassail, in order to ensure their thriving — hence the tradition of wassailing apple trees.

According to the OED the word is first recorded in Geoffrey of Monmouth circa 1140. It derives from the Old Norse ves heill via the Old English wes hál, literally ‘be in good health’ or ‘be fortunate’ and the Middle English wæs hæil.