Fishtailing

[Dianne]

Similar to a wedgie. The [perpetrator] [sneaks] up behind the victim and quickly inserts his fingers in between [the victims] ass cheeks, hitting the sensitive skin and putting tension on the victims cheeks and anus. If done correctly can be extremely painful, however should only be done in a joking manner among friends.

[Justin Bieber]

When girls who go to a [uniform] school [roll] their [kilts] wrong, the pleats at the back stick out.