The pre/post excuse phenomena is the tendency of players of any game to excuse their losses through a set of (highly predictable) reasons.
The pre-excuse is announced before the game, and is normally some pervading "negative condition" on the pre-excuser's end. Usually a gentle reminder will be dropped about the condition AFTER the game, whether the player who gave the pre-excuse won or lost. In the event of a loss he/she would attribute their failure to the pre-arranged reasons. If the game was a win, they would imply that they won in spite of the horrible handicap and are twice as awesome as a result.
Some common examples:
And of course lets not forget the post-excuse. This occurs after the player takes a loss, and often comes in the form of some act which occurs in-game which destroys the player's chance of winning.
So in closing, pre and post excuses are used by players to justify their horrible lack of skill. If you start gaming be sure to develop a vast stable of excuses, that way people can be annoyed by how whiny you are, and you will make hundreds of internet friendships.
If there are any glaring grammar/spelling problems just send me a message, I would have kicked this node's face in if I wasn't watching MXC while I wrote this. :(