The preparation before some kind of big kick-ass event. Kind of like tailgating. Can be applied to pre-gaming a party or a...well...orgy.
Dude 1: Hey, are you goin' to the soccer game today? I hear it's at 7.
Dude 2: No bra! I'm goin' to the beforgy at Brian's. It starts at 6 and there's guaranteed to be lot's of shitty vodka and slutty freshman girls.
Dude 1: Man I'm there like mohair.