Etymology
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holy (adj.)
Origin and meaning of holy

Old English halig "holy, consecrated, sacred; godly; ecclesiastical," from Proto-Germanic *hailaga- (source also of Old Norse heilagr, Danish hellig, Old Frisian helich "holy," Old Saxon helag, Middle Dutch helich, Old High German heilag, German heilig, Gothic hailags "holy"), from PIE *kailo- "whole, uninjured" (see health). Adopted at conversion for Latin sanctus.

The primary (pre-Christian) meaning is not possible to determine, but probably it was "that must be preserved whole or intact, that cannot be transgressed or violated," and connected with Old English hal (see health) and Old High German heil "health, happiness, good luck" (source of the German salutation Heil). Holy water was in Old English.

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 has been used as an intensifying word from 1837; in expletives since 1880s (such as holy smoke, 1883, holy mackerel, 1876, holy cow, 1914, holy moly etc.), most of them euphemisms for holy Christ or holy Moses. Holy Ghost was in Old English (in Middle English often written as one word). 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.

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Heligoland 
island in the North Sea off Germany, from the same source as German heilig "holy" (see holy), in reference to an ancient shrine there.
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Holy Land 
"western Palestine, Judaea," late 13c., translating Medieval Latin terra sancta (11c.).
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halibut (n.)
large flatfish, early 15c., perhaps from hali "holy" (see holy) + butte "flatfish" (see butt (n.4)). Supposedly so called from its being eaten on holy days (compare cognate Dutch heilbot, Low German heilbutt, Swedish helgeflundra, Danish helleflynder).
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holiness (n.)
Middle English holinesse, from Old English halignis "state or character of being holy, sanctity, religion; holy thing;" see holy + -ness. Compare Old High German heilagnissa. As title of the Pope (mid-15c. in English), it translates Latin sanctitas (until c. 600 also applied to bishops).
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hollyhock (n.)
mid-13c., holihoc, probably from holi "holy" (see holy) + hokke "mallow," from Old English hocc, a word of unknown origin. Another early name for the plant was caulis Sancti Cuthberti "St. Cuthbert's cole." Native to China and southern Europe, the old story is that it was so called because it was brought from the Holy Land.
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unholy (adj.)
Old English unhalig, "impious, profane, wicked," from un- (1) "not" + halig (see holy). Similar formation in Middle Dutch onheilich, Old Norse uheilagr, Danish unhellig, Swedish ohelig. In reference to actions, it is attested from late 14c. Colloquial sense of "awful, dreadful" is recorded from 1842.
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holiday (n.)
1500s, earlier haliday (c. 1200), from Old English haligdæg "holy day, consecrated day, religious anniversary; Sabbath," from halig "holy" (see holy) + dæg "day" (see day); in 14c. meaning both "religious festival" and "day of exemption from labor and recreation," but pronunciation and sense diverged 16c. As an adjective mid-15c. Happy holidays is from mid-19c., in British English, with reference to summer vacation from school. As a Christmastime greeting, by 1937, American English, in Camel cigarette ads.
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kaddish (n.)
doxology of the Jewish ritual, 1610s, from Aramaic (Semitic) qaddish "holy, holy one," from stem of q'dhash "was holy," ithqaddash "was sanctified," related to Hebrew qadhash "was holy," qadhosh "holy." According to Klein, the name probably is from the second word of the text veyithqaddash "and sanctified be."
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