An Etymological Dictionary of the German Language/heilig
heilig, adj., ‘holy, sacred, inviolable,’ from the equiv. MidHG. heilec, OHG. heilag, adj.; comp. OSax. hêlag, AS. kâleg, E. holy, OIc. heilagr, adj.; all have the common meaning, ‘sanctus.’ In Goth. only is the adj. unknown (yet hailag occurs in a Goth. Runic inscription); the earlier old heathen form weihs (see weihen) was used instead. The development of meaning in heilig from the subst. Heil is not quite clear. Is the word Heil used in a religions sense? Comp. OIc. heill, ‘favourable omen,’ OHG. heilisôn, ‘to augur,’ OIr. cel, ‘augury’?.