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.
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 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.
Obviously, the word “holy” has quite a history and didn’t always mean what it does today – at least in the common vernacular. Scripturally speaking, we see much more consistency as to its meaning.
The definitions above hint or point to the words meaning but don’t come out and say it so I will. The word “holy” is to be “set apart”. It designates sacred things, persons, etc. because these are set apart from others in one fashion or another. It is a word that says something both about what it is describing as well as what is not being described with that word.
When talking about God, this means God is inherently separate from human beings. This becomes all the more obvious when considering God’s other characteristics.
- He is infinite. We are finite.
- He is immortal. We are mortal.
- He is spirit. We are created material.
- He is holy. We are inherently unholy.
…I think you start getting the idea.