From an unused root (meaning to frighten); frightful -- terrible.
H366. ayom
אָיֹם adjective terrible, dreadful — terrible, of Chaldeans הוּא וְנוֺרָא אָיֹם Habakkuk 1:7; of dignified woman, awe-inspiring כַּנִּדְגָלוֺת אֲיֻמָּה Song of Solomon 6:4, 10.