It is a formal declaration issued in the event that you will pwn someone at a later date and time if you cannot presently do it. A postpwnment is normally declared when you yourself have been pwned, but do not have the time to retaliate.
Yesterday Dave totally pwned me in CS, but I was too damn tired, so I issued a postpwnment until next week.
by Dr.Gutin January 31, 2006