A rare situation in a game of eight ball. The first player or team must first of all have the second team one ball away from losing pants down; i.e. the first team must have potted all of their balls except for the black ball, and the second team must have all seven of their balls left on the table. If the second team then comes back from this position to win the game, the first team is reverse pantsed, and must suffer the same penalty as regulation pants down, namely to walk around the table with their pants down (or skirt up).
Note that reverse pantsing only occurs when the team pots all seven of their balls and the black ball to win. If the team who was almost pants down wins because the other team followed the black ball with the cue ball, reverse-pantsing does not apply.
Dang, haven't seen a reverse pantsing like that in a while.