wassail

Old English wesan, wæs, wæron 1st and 3rd person singular of wesan "to remain," from Proto-Germanic *wesanan (source also of Old Saxon wesan, Old Norse vesa, Old Frisian wesa, Middle Dutch wesen, Dutch wezen, Old High German wesen "being, existence," Gothic wisan "to be"), from PIE root *wes- (3) "remain, abide, live, dwell" (cognates Sanskrit vasati "he dwells, stays;" compare vestal). Wesan was a distinct verb in Old English, but it came to supply the past tense of am. This probably began to develop in Proto-Germanic, because it is also the case in Gothic and Old Norse. See be.

Old English hælþ "wholeness, a being whole, sound or well," from Proto-Germanic *hailitho, from PIE *kailo- "whole, uninjured, of good omen" (source also of Old English hal "hale, whole;" Old Norse heill "healthy;" Old English halig, Old Norse helge "holy, sacred;" Old English hælan "to heal"). With Proto-Germanic abstract noun suffix *-itho (see -th (2)).

Of physical health in Middle English, but also "prosperity, happiness, welfare; preservation, safety." An abstract noun to whole, not to heal. Meaning "a salutation" (in a toast, etc.) wishing one welfare or prosperity is from 1590s. Health food is from 1848.

salutation in greeting, c. 1200, from Old Norse heill "health, prosperity, good luck," or a similar Scandinavian source, and in part from Old English shortening of wæs hæil "be healthy" (see health; and compare wassail).

The interj. hail is thus an abbreviated sentence expressing a wish, 'be whole,' i. e., be in good health, and equiv. to L. salve, plural salvete, or ave, plural avete .... [Century Dictionary]