Middle English holi, from Old English halig "divine, sacred, to be revered or worshipped; consecrated, sacred; godly, perfect in religious character and practice of devotion; ecclesiastical," from Proto-Germanic *hailaga-. This is reconstructed (Watkins) to be from PIE *kailo- "whole, uninjured."
Used in Old English, after the conversion, for Latin sanctus. From the early forms are derived hallow (n.) "a saint," hallow (v.) "to sanctify," halibut, Halloween.
The primary (pre-Christian) meaning is uncertain, but probably it was "that must be preserved whole or intact, that cannot be transgressed or violated," and the word shares a PIE root with Old English hal (see health) and Old High German heil "health, happiness, good luck" (source of the German salutation heil).
In Middle English holy also was used of pagan gods, the Hebrew temple or tabernacle, and of the Quran and Muslim doctrine.
Holy is stronger and more absolute than any word of cognate meaning. That which is sacred may derive its sanction from man ; that which is holy has its sanctity directly from God or as connected with him. Hence we speak of the Holy Bible, and the sacred writings of the Hindus. He who is holy is absolutely or essentially free from sin; sacred is not a word of personal character. The opposite of holy is sinful or wicked; that of sacred is secular, profane, or common. [Century Dictionary, 1895]
Holy water was in Old English. Holy Ghost was in Old English (in Middle English often written as one word). Holy Father for "the pope" is attested by early 13c.
Holy Land is by early 13c. as "the kingdom of Heaven, paradise;" by c. 1300 as "western Palestine, Judaea," the setting of much of the New Testament.
Holy has been used as an intensifying word and in expletives from mid-19c., such as holy smoke (1871), holy mackerel (1865 as by the holy mackerel; 1860 as holy St. Mackerell), holy cow (1910), holy moly, etc., most of them likely euphemisms for holy Christ! or holy Moses!
Holy League is used of various European alliances; the Holy Alliance was that formed personally by the sovereigns of Russia, Austria, and Prussia in 1815; it ended in 1830.
Germanic cognates include Old Norse heilagr, Danish hellig, Old Frisian helich "holy," Old Saxon helag, Middle Dutch helich, Old High German heilag, German heilig, Gothic hailags "holy."
