A common occurrence during sequestering within a confined area/group. This may include a vacation group, a work place, classroom, etc.. The point is being semi isolated from the general population. Can occur for women or men, however most likely for men.
A 391 begins when a person enters above mentioned semi sequestered environment. They are then presented with the local selection of opposite sex individuals. Being horny, the individuals are quickly assessed and the best one (or few) are rated as 3's, max.
Because of the sequestering, and horniness, the 3's quickly morph into 9's. Alcohol is generally involved, as well as close proximity. At this point a useless power struggle may ensue (especially among men) to get the attention of the hottest '9'. A pointless exercise.
Upon further reflection, and especially upon entering the general populace and being again presented with the abundance of the opposite sex, the previously rated 9's quickly become 1's.
The horror of the situation is quickly apparent to all that have come under the influence of the 391 phenomenon. Shame spirals ensue.
Friend 1: Dude! How was the Contiki trip to Europe??
Friend 2: It was nice. 391's though bro, almost got into a fist fight for a while.
Friend 1: Drag.
Friend 2: Thank god we eventually made it to Eastern Europe and I learned the folly of my ways...