Holy means belonging to God, or set apart for religion. A holy book kept on a high shelf, holy water in a stone bowl by the church door, a holy day when the shops stay shut.
A place can be holy — holy ground, the Holy Land — and so can a person who has given their life to God. Capital letters mark the fixed names: the Holy Bible, the Holy Spirit, the Holy Grail. In speech it turns into an exclamation of shock, holy cow, and into an insult when someone is holier-than-thou, meaning smugly certain they are the better person. Careful with people: calling someone holy says they are devoted to God, not that they are simply kind. The near-twin sacred works for anything treated as untouchable — a sacred rule, a sacred memory — where holy pulls the sentence towards religion.
1adjectiveholyConnected with God or with religion, and treated with deep respect.
“They took off their shoes because the ground was holy.”
2adjectivedevotedDevoted to God; living a religious life.
“He was a quiet, holy man who prayed at four every morning.”
3adjectiveexclamationsUsed in exclamations to show shock or amazement.
“Holy cow, look at the size of that dog.”
Too few real examples of this word to estimate which sense is commonest.
Also accepted in games: sacred
Old English hāliġ meant holy and sacred, and also healthy and sound. It grew out of Proto-Germanic *hailagaz, from *hailaz, whole and healthy, and further back a Proto-Indo-European root meaning intact and complete. So holy and whole are the same idea twice: what is holy is unbroken.
Proto-Germanic
*hailagaz — holy, sacred
Proto-West Germanic
*hailag
Old English
hāleġ
Middle English
ali
English, today
holy
Built from
From Middle English ali, aly, hale, halege, hali, halȝe, hayly, holi, holia, holie, holiȝ, holy, hooly, oli, oly, wholy, woly, from Old English hāleġ, hāliġ (“holy, sacred; healthy, sound”), from Proto-West Germanic hailag, from Proto-Germanic hailagaz (“holy, sacred”), from Proto-Germanic hailaz (“healthy; whole”), from Proto-Indo-European kéh₂ilos (“intact, whole; complete; hale, healthy, sound”), equivalent to whole + -y and a a doublet of - later a doublet of wholly. 2 Cognate with Scots halie, haly (“holy”), yol holly, holy (“holy”), stq helig (“holy”), gsw, Dutch and German heilig (“holy, sacred”), cim hòolig (“holy”), lb helleg (“holy”), wym haeliḱ, haliky, hȧlikjy, hǡliḱ (“holy”), Yiddish הייליק (“holy”), Danish and Norwegian Bokmål hellig (“holy”), Faroese heilagur (“holy”), Icelandic heilagur, helgur (“holy”), Norwegian Nynorsk heilag, heilag’u (“holy”), Swedish helig (“holy”), Gothic 𐌷𐌰𐌹𐌻𐌰𐌲𐍃 (“holy”); also Latin cael, caelus, caelum, coelus, coelum (“heaven; sky; climate, weather”), be цэ́лы (“entire, whole”), Bulgarian цял (“entire, full, intact, whole; real, regular, veritable”), Czech and Slovak celý (“whole”), mk цел (“entire, whole”), Polish cały (“entire, whole”), Russian це́лый (“entire, whole”), Serbo-Croatian це̏о, ци̏јел, ци̏о, cȅo, cȉjel, cȉo (“entire, whole”), Slovene cel (“whole”), Ukrainian ці́лий (“entire, whole”). More at whole.
Etymology from Wiktionary, CC BY-SA