It's like 'man'. "Hey what's up man?" ... "Hey what's up dude?" although that way of addressing someone is informal you can say "This (gentle)man is acquainted with you?" and it will not taken with offence but "This dude is acquainted with you?" can seem hostile if you are in a formal situation. The female equivilant is 'dudette' ^^

Hellopanda77 and Sellymoo said it..."Dude" is an American English slang term for "man" but not man as in the gender. It's used when you don't call the person by name and by teenagers and some early college-age people. It's a term that comes from California surfers. Often you'll hear it in American shows from the 1960's, and especially, the 1970's. Some internet sites say it's a cowboy term, which isn't right. Also, it has nothing to do with any bad sexual meanings like some internet sites like to say. "Dudette" is the word used when talking to a girl. Like hellpanda77, I'm from Southern California where this word is still used today all of the time on the beach.

we also use "dude" as an exclamation, especially in calif, where i'm from ;) "dude! it's so cold outside!" "dude! i almost got killed yesterday." "dude, you gotta see this movie"

Ｊｕｓｔ　ａｓ　Ｓｅｌｌｙｍｏｏ　ｓａｉｄ．

Dude (Slang) means ''Kanka, adamım,dostum'' in Turkish :D