1. wart on male horse's butt with hairs growing out of it
2. Stupid Language for anything along the lines of you, he, she, it, huh?, etc.
3. a noun
4. basically anything idiotic or inappropariate
a dude is a guy. that's basically all there is to it. sometimes, you might just use it to address anyone though. it's just a way to acknowledge people.
Dude 1: "Dude, she is hot!"
Dude 2: "Um, dude, no she isn't."