hallow v.

also Hallow-e'en, Hallow e'en, "last night of October (the eve of All Saints Day) as a popular holiday," 1781, in a Scottish context, a Scottish shortening of Allhallowe'en, All Hallows even, etc., 1550s, "the evening before All-Hallows." This is from otherwise-obsolete hallow (n.), in Middle English halwe, "holy person, saint," from Old English halga, which is from the source of hallow (v.). Also see even (n.), and compare hallows.

All-Hallows is Middle English al-halwe, late Old English ealra halgan "all saints, the saints in heaven collectively," also the name of the feast day and of individual churches. The date Oct. 31 is described as alle halwe eue by c. 1300. Hallow-day for "All-Saints Day" is from 1590s; earlier was halwemesse day (late 13c.).  Hallowtide (15c.) was the first week of November.

The last night of the year in the old Celtic calendar, where it was Old Year's Night, a night for witches. A pagan holiday given a cursory baptism. The word and the magical lore about the date were popularized by Burns' poem (1785, and he attached a footnote explaining it), but it probably dates to 17c. in Scotland and is the name of a tune in 1724. The tune is mentioned again in an English-Scots songbook ("The Chearful Companion") in 1783, and Burns was not the first to describe the customs in print.

Hallow-E'en, or Holy Eve, is the evening previous to the celebration of All Saints. That it is propitious to the rites of divination, is an opinion still common in many parts of Scotland. [John Main, footnote to his poem "Hallow-E'en," Glasgow, 1783]

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.