The story
Origin unclear. One theory is that it originates from the word duly, as in “duly appointed [a cadet]” (part of the official acceptance). Another claims that the term is derived from Ancient Greek δοῦλος (doûlos, “slave”) + -ie. Apparently dates to the first class of the Academy, in 1955.
Etymology from Wiktionary, CC BY-SA 4.0.