"Dude" is a slang term that is primarily used in American English to refer to an individual, typically a young male.
It evolved into mainstream use within American culture to refer to males in the 1970s. The first instances of variations on the word are dated back to the 1500s, used to refer to clothing. However the first instance of its use as a slang term for an individual dates back to 1876, in which is was used in publication to mockingly refer to the way a woman was dressed.
By the 1880s, it had evolved into a popular term among the press to refer to a man who was unusually obsessed with keeping up with their appearance. The term "duded-up" also was used as an adjective for the same purpose. It also became a term used often by cowboys to refer to city-dwellers.
In 1885, Buford Tannen and his gang openly mocked a stranger's attire. Following this, Buford asked the strangely dressed teenager "What's your name, dude?!", as a way to mock the teenager's attire, as well as the fact that he was most likely a city-slicker. The stranger responded that his name was Clint Eastwood, and then exclaimed "You're Mad Dog Tannen!" Buford became irate, referring to Clint as "duded-up egg-sucking gutter-trash".