An awkward action done at the end of a date. Usually associated with a fist pump or a high five. The action is totally unexpected to the receiver of the mark jackson, and comes natural to the person doing it. The person giving the mark jackson is blissfully unaware of the action, and as a result has no idea that the action resulted in a total loss of social respect from the person's peers.
John: Did you hear what happened at the end of Heather's date?
Connor: No, did she kiss him? It was only the first date
John: Dude she told me that he said good bye and gave her a fist pump!
Connor: Oh man she just got Mark Jacksoned!!!