The concept and title originates from Moshe Rabbeinu in parshat v'Zot HaBracha 33:1, which calls him "Ish HaElokim". It translates as The "Man/G-d". It is referring to a true prophet of G-d as explained in Sefer HaChinuch, Mitzvah 517, Shoftim.
This is also how it is explained by Rabbi Meir Ibn Gabbai in Sefer Avodat HaKodesh, volume 2: Perek 5 in the name of the sage, Rabbi Yosef ben HaRav HaMekubal, Rabbi Shem Tov ben Shem Tov.
The concept is developed further by the Magid of Mezeritch in his Likkutei Amarim, chapter 29 beginning with the words, "Make for yourself two trumpets of silver..." The Magid explains that the word "trumpets" (חצוצרות) is to be understood "two halves" (חצי צורות). That a true prophet is actually half man and half G-d.
And this is in keeping with the words of the Sefer HaChinuch which explains such a person, the true prophet, is an actual agent of G-d, meaning "shaliach" as it is understood in Halacha. That means that the shaliach, the legal agent or representative actually represents the one who has empowered him. He speaks for him and acts with all his authority and power. When the shaliach obligates the one that empowered him, the one that empowered him is fully responsible to fulfill the commitments made by the shaliach.
This idea is also brought by Rabbi Chaim of Volozhin in Sefer Nefesh HaChaim in relation to the idea that the Shechina spoke from the midst of Moshe's throat. See Nefesh HaChaim, Sha'ar 3, Perek 13 and Likutei Ma'amarim, Ma'amarim 14.
In the same way, HaKadosh is the also one of the upper levels of prophecy as taught by Rabbi Pinchas ben Yair in Avodah Zara 20a beginning with the words, "Torah brings one to watchfulness...". The level just before "Ruach HaKodesh" is called "Kedusha". One who has attained this level is called "HaKadosh".
This is also explained by Rabbeinu Bachai in his commentary to Devarim, parshat v'Zot HaBracha 33:8.