An Etymological Dictionary of the German Language/Annotated/heilig
heilig, adjective, ‘holy, sacred, inviolable,’ from the equivalent Middle High German heilec, Old High German heilag, adjective; compare Old Saxon hêlag, Anglo-Saxon kâleg, English holy, Old Icelandic heilagr, adjective; all have the common meaning, ‘sanctus.’ In Gothic only is the adjective unknown (yet hailag occurs in a Gothic Runic inscription); the earlier old heathen form weihs (see weihen) was used instead. The development of meaning in heilig from the substantive Heil is not quite clear. Is the word Heil used in a religions sense? Compare Old Icelandic heill, ‘favourable omen,’ Old High German heilisôn, ‘to augur,’ Old Irish cel, ‘augury’?.