If the word holy is an adjective, what is it the adjective of?

it modifies whatever word, clause or phrase following it. Ghost or cow for example. Holy has no relationship to hole. “Holey” is what you’re lookin for.

…moly!

Holy is an adjective like red and rough and square are adjectives. They stand alone, they are not tied to a root word.

Got it.

Holy:
Origin
Old English hālig, of Germanic origin; related to Dutch and German heilig, also to whole.

Source: Oxford Online Dictionary

Holy lexicon @flo , an adjective adds meaning to a noun cf. little red car.

http://www.etymonline.com/index.php?term=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.

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.

https://www.youtube.com/watch?v=8xZksYrWai8

Rolling Stones The Last Time 1965

Well I told you once and I told you twice
But ya never listen to my advice
You don’t try very hard to please me
With what you know it should be easy

I think that you are thinking of of the word wholly, which is an adverb derived from the word whole.

I believe you are confusing adverbs and adjectives. Adverbs frequently end in -ly, adjectives do not. Holy would be an adjective for whatever you want to make it an adjective for…it describes something.

How about:
https://www.englishforums.com/English/WordsAdjectivesLy/mzb/post.htm