How on earth did you find that?
My friend, have you noticed in these few years that I believe in particular attention to words? :)
It was pretty easy: Etymonline, which is part of my few core references as "dictionary of meaning" (as opposed to "dictionary of use"), gives the first clues at https://www.etymonline.com/search?q=dang . Then I looked for the original text and checked the details.