A fork of Rural Dictionary
A double fisting that incorporates both anal and vaginal penetration. Fists are then pumped in and out in manner similar to that of pistons in a engine.
I gave that filthy sow the piston fisting of her life.
The art of shoving one whole fist in a girl's arse and the other in her vagina and ramming them in and out alternately (like a piston system)
I was piston fisting her til she blead