The term dates back to the 1300s, when the word dudde referred to a cloak or mantle of coarse cloth. Over time, it came to refer to shabby clothing, and eventually acquired a more neutral meaning of simply “clothes.” The earlier sense of “ragged” or “inferior” may also be reflected in the term dud, denoting something that fails to function (such as a missile, torpedo, or a business enterprise).
It is not to be confused with the word “dude,” as in “Hey dude, surf’s up!”