Wassail was originally spelled Wæs hæl ("Be hale/healthy!" [pronounced with short a, to rhyme with lass gal]). The "modern" spelling became current in the late 12th/early 13th century. It is not only an imperative to be well, but also the drink used to toast each other's health in the bleak midwinter. Ralph Holinshed (1529-1580) in his Chronicles quotes a story from Geoffrey of Monmouth's (c.1100-c.1155) Historia Regum Britanniæ (History of the Kings of Britain):
A great supper therefore was prepared by Hengist at the which it pleased the king [Vortigern] to be present, and he [Hengist] appointed his daughter [Rowen], when every man began to be somewhat merry with drink, to bring in a cup of gold full of good and pleasant wine, and to present it to the king saying; “Wassail.” Which she did in such comely and decent manner, as she that knew how to do it well enough, so as the king [Vortigern] marveled greatly thereat, and not understanding what she meant by that salutation, demanded what it signified. To whom it was answered by Hengist, that she wished him well and the meaning of it was, that he should drink after her,...Currently, many who make wassail start with red wine, but originally it was based on heated cider or ale with spices and fruit thrown in. Ale or cider mixed with sugar, cinnamon, and nutmeg was common. The hot concoction would have toasted bread tossed on it to sop up the liquid for easy consumption. A traditional carol alludes to this:
Wassail! wassail! all over the town,If you are interested, there are countless recipes online for wassail. For a modern take on old recipes, Alton Brown is always reliable. A recipe that sticks more to its roots can be found at Nourished Kitchen. For an inauthentic recipe that attempts to make wassail easy for the modern cook, you could do worse than Gode Cookery.
Our bread it is white and our ale it is brown;
Our bowl it is made of the white maple tree;
With the wassailing bowl, we'll drink to thee.