Usually found in a hardcore show, a pile-on is when everyone rushes from the pit to the front of the stage and jumps on the shoulders of the person in front of them. This is done mostly during a sing-along where the vocalist gives up the mic to the crowd, much like a gang vocal.
Dude that pile-on during Mutiny was fucking croosh!
A Sexual Position... In Which Multiple Bodies Insert Into Each Other... The More Participants The Higher The Pile
I Was So Far Down In The Pile It Hurt... My Ribs Started To Crush
OMFG Check The Size Of That Pile-On Its A Mofo!