The Collaborative International Dictionary of English v.0.48:

Wassail \Was"sail\, n. [AS. wes h[=a]l (or an equivalent form in
   another dialect) be in health, which was the form of drinking
   a health. The form wes is imperative. See Was, and
   Whole.]
   [1913 Webster]
   1. An ancient expression of good wishes on a festive
      occasion, especially in drinking to some one.
      [1913 Webster]

            Geoffrey of Monmouth relates, on the authority of
            Walter Calenius, that this lady [Rowena], the
            daughter of Hengist, knelt down on the approach of
            the king, and, presenting him with a cup of wine,
            exclaimed, Lord king waes heil, that is, literally,
            Health be to you.                     --N. Drake.
      [1913 Webster]

   2. An occasion on which such good wishes are expressed in
      drinking; a drinking bout; a carouse. "In merry wassail he
      . . . peals his loud song." --Sir W. Scott.
      [1913 Webster]

            The king doth wake to-night and takes his rouse,
            Keeps wassail.                        --Shak.
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            The victors abandoned themselves to feasting and
            wassail.                              --Prescott.
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   3. The liquor used for a wassail; esp., a beverage formerly
      much used in England at Christmas and other festivals,
      made of ale (or wine) flavored with spices, sugar, toast,
      roasted apples, etc.; -- called also lamb's wool.
      [1913 Webster]

            A jolly wassail bowl,
            A wassail of good ale.                --Old Song.
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   4. A festive or drinking song or glee. [Obs.]
      [1913 Webster]

            Have you done your wassail! 'T is a handsome, drowsy
            ditty, I'll assure you.               --Beau. & Fl.
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The Collaborative International Dictionary of English v.0.48:

Wassail \Was"sail\, v. i.
   To hold a wassail; to carouse.
   [1913 Webster]

         Spending all the day, and good part of the night, in
         dancing, caroling, and wassailing.       --Sir P.
                                                  Sidney.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Wassail \Was"sail\, a.
   Of or pertaining to wassail, or to a wassail; convivial; as,
   a wassail bowl. "Awassail candle, my lord, all tallow."
   --Shak.
   [1913 Webster]

   Wassail bowl, a bowl in which wassail was mixed, and placed
      upon the table. "Spiced wassail bowl." --J. Fletcher.
      "When the cloth was removed, the butler brought in a huge
      silver vessel . . . Its appearance was hailed with
      acclamation, being the wassail bowl so renowned in
      Christmas festivity." --W. Irving.

   Wassail cup, a cup from which wassail was drunk.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wassail
    n 1: a punch made of sweetened ale or wine heated with spices
         and roasted apples; especially at Christmas
    v 1: celebrate noisily, often indulging in drinking; engage in
         uproarious festivities; "The members of the wedding party
         made merry all night"; "Let's whoop it up--the boss is
         gone!" [syn: revel, racket, make whoopie, make
         merry, make happy, whoop it up, jollify, wassail]
    2: propose a toast to; "Let us toast the birthday girl!"; "Let's
       drink to the New Year" [syn: toast, drink, pledge,
       salute, wassail]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "wassail":
   bacchanal, bacchanalia, bacchanalian, bat, bender, binge, bout,
   bust, carousal, carouse, celebration, compotation, debauch,
   drinking bout, drunk, drunken carousal, frolic, guzzle, hell,
   high jinks, jag, orgy, potation, pub-crawl, revel, revelment, riot,
   roister, skylarking, soak, spree, symposium, tear, toot, whoopee