The word 'dude' is one of the most interesting words in American-English slang. One very officious dictionary defines the term as:
n. 1. [Invented term, 1883], 1. a dandy; fop.
2. [Western Slang], a city fellow or tourist, especially an Easterner.
For most modern Americans, however, even this definition requires some explaining. More likely than not, one would need to define 'dandy' and 'fop', and also explain that 'Westerner' and 'Easterner' relate to parts of the United States, as opposed to regions of the globe, and also that the East refers specifically to the stereotype of the industrial Northeast, and the Southwest to stereotypical cowboy characteristics.
'Dude' is an exceptionally versatile word. Depending on the tone used, it can span a variety of meanings. It can be an exclamation of delight, an expression of dark horror, a cry of anger, a paean of love, and many other things besides. This word is not for use by the novice, and should be considered carefully if heard, to avoid misunderstandings. 'Dude' can be used many times in a single sentence, with only the tone and familiarity with the speaker to guide the listener as to the conversation's meaning.
American 1: Dude [greeting], yesterday I talked to dude [mutual aquaintance], and he said 'Dude [realisation]! I haven't seen dude [reference to American 2] in forever! Next time you see him, tell him dude [greeting] for me.
American 2: Dude [appreciation]. That dude [mutual aquaintance] rocks.
American 1: Dude. [understanding and agreement - usually accompanied by head nodding]
There are many different stories surrounding the word's origins. One such story claims that it was created by Oscar Wilde, as a combination of the words 'dud' and 'attitude'. Another claims that the word was formed by cowboys in the West to refer (unfavourably) to city dwellers1. Of course, none of these possible origins change its current complex maze of meanings one whit.
It is recommended that 'dude' is used by advanced slang speakers only. Any who choose to use this word do so at their own risk.