Origin: 1175–1225; Middle English was-hail, equivalent to was be (Old English wæs, variant of wes, imperative of wesan to be; akin to was) + hailhale1, in good health (< Old Norse heill hale); replacing Old English wæs hāl be hale or whole. See whole, heal
c.1140, from O.N. ves heill "be healthy," a salutation, from ves, imperative of vesa "to be" (see was) + heill "healthy" (see health). Use as a drinking phrase appears to have arisen among Danes in England and spread to native inhabitants. A similar
formation appears in O.E. wes þu hal, but this is not recorded as a drinking salutation. Sense extended c.1300 to "liquor in which healths were drunk," especially spiced ale used in Christmas Eve celebrations. Meaning "a carousal, reveling" first attested 1602. Wassailing "custom of going caroling house to house at Christmas time" is recorded from 1742.