1883, "the dress, manners, and social peculiarities of the class known as dudes" [Century Dictionary], from dude + -ism.
The dude possesses in his outward appearance and bearing all the attributes of a gentleman, excepting, perhaps, that of manliness. His dress is unostentatious in its perfection, its only loud notes being a pair of white gaiters, which are believed to be going out already in obedience to the unwritten code of dudeism. Why the dude feels any interest in life is not clear--he does not look as if he enjoyed it. There is a certain introspective earnestness in his bearing that reminds one of the theological student, and perhaps the prevailing high collar strengthens the resemblance. [Phrenological Journal, July 1883]