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.
Mike: Damn I was walking in my living room yesterday, and Dana fucking fishtailed me so fucking hard.
Dave: Damn, that sucks dude. I hate Fishtailing