Following Thomas Jacobs' excellently researched answer which traced the term to Ed Greenwood in a Dragon magazine article and in the Powers of Faerûn book , I took the matter to Ed Greenwood himself, who is active on twitter, and asked him for the origin of the term.
@TheEdVerse Hey there. A question has arisen in the RPG Stackexchange site about the origin of the "Ardragon" rank for Zhentarim agents. Investigation has traced the source to a book and an article you wrote a while back. Could you tell us more about that?
Ed Greenwood's 2-part reply here and here:
Sure! Happy to! "Ardragon" is a corruption (over centuries of usage) of "Irondragon," which began as a praising description of a hardy, veteran, experienced, calm-in-crisis warrior, in use around the Sea of Fallen Stars. Gradually, in the early Moonsea settlements, it became a rank/title, and was in use (as "ardragon") in Zhentil Keep before the rise of the Zhentarim, who adopted it as they took over command of the city's existing military (in existence to defend the city against raids from ogres, flind, orcs, hobgoblins, gnolls, and rival cities).
Now, I don't know if this etymology has always been in his notebooks when designing the Zhentarim and the other cultures around the Sea of Fallen Stars, or whether he whipped it up for our edification when I asked, but as far as I'm concerned, this is Word of God, and answers the question. Hurrah for Twitter and hurrah for Ed Greenwood, and his cheerful accessibility.