holy (adj.) Look up holy at Dictionary.com
Old English halig "holy, consecrated, sacred, godly," from Proto-Germanic *hailaga- (cognates: 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 (such as holy smoke, 1883, holy mackerel, 1876, holy cow, 1914, holy moly etc.), most of them euphemisms for holy Christ or holy Moses.