A phrase you say when:
1) People suggest for you to do something completely retarded.
2) You don't really believe what someone has said.
3) Someone says something stupid that makes no sense.
4) Someone is making fun of you.
Also can be used frequently with cool thing
. The phrase becomes really effective if you use hand motions to interpret it. The most common one is moving your hand in a roller coaster fashion: Move it slowly up like you are climbing the first hill on a coaster for the first two words, and draw them out saying them in a high pitched voice....Yeah Maaaaaaaaaaannnn....Then for the next word, still in a high pitched voice, act like you are at the top of the hill and stop for a second....Yooouuuu(Pause)....Then plummet down the hill with the last word saying it real fast and in a deep voice....Right....And thus you have the hand motions.