The upnod is a form of public acknowledgement among guys that started in the 1990s. Instead of nodding their heads down they will move their heads up, usually with an exclamation such as "sup" (a contraction of what's up?), or "hey" or "howyadoin" (a contraction of How are you doing?).
The upnod tells the other that the upnodder believes himself to be a socially adept person. It does not imply any belief about the other person's social situation, other than the fact the two are within close enough proximity to require such social lubrication.
The upnod was popularized by U.S. beer commercials ("Budweiser", I believe) shown on televisions around Super Bowl season in the 2002-2004 timeframe. The beer commercials legitimized such inter-masculine forms of communication. They also had the effect of requiring social adepts to find other forms of greeting. We are still casting about for these forms. Should you have any suggestions, please /msg the author. Hey, what's up?
jessicapierce says re March 10, 2004: HA you do the upnod.