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Old English halig "holy, consecrated, sacred, godly," from Proto-Germanic *hailaga- (cf. Old Norse heilagr, Old Frisian helich "holy," Old Saxon helag, Middle Dutch helich, Old High German heilag, German heilig, Gothic hailags "holy"). 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; used in expletives since 1880s (e.g. holy smoke, 1883, holy mackerel, 1876, holy cow, 1914, holy moly etc.), most of them euphemisms for holy Christ or holy Moses.
the power, being, or realm understood by religious persons to be at the core of existence and to have a transformative effect on their lives and destinies. Other terms, such as holy, divine, transcendent, ultimate being (or reality), mystery, and perfection (or purity) have been used for this domain. "Sacred" is also an important technical term in the scholarly study and interpretation of religions