Q. Who invented the term High Holydays?
But neither word is a literal translation of the Hebrew yom kadosh, a sacred day; in English, “holy” derives from the Middle English “hool”, which means not “sacred” but “whole” or “excellent”. Yom kadosh is found in Hebrew in the T’nach in various forms, but generally with reference to Shabbat (e.g. Isa. 58:13).
The now well known phrase, yamim nora’im (“awesome days”) is not Biblical, nor am I aware of it being found in the Talmud; Professor Ismar Elbogen, author of the monumental “Jewish Liturgy in its Historical Development”, believed it was medieval and reflected a change in the mood of Rosh HaShanah from a predominantly joyous celebration to a more subdued day reflecting times of persecution.
Despite Elbogen, however, the traditional liturgy says, “This day is holy to our Lord”.
It is worth adding that there are always people who get things wrong, and I have heard someone who should know better speaking of the yamim narronim – and narronim means “idiots”!