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1.
To fist a girl's vagina or ass or a Man's ass so hard and fast that either time explodes or the universe comes apart at the seams. Enducing lulz.

Powerfisting must be used in conjuction with the "powerfist shout" in order to reach maximum power and lulz.
Wendy: Yeah, don't stop...more
Gary: How about a powerfisting?
Wendy, Yeah..do it..
Gary POOOOOOOWERFIIIIIIIIIIST!

*Universe collapses*
by Baron von Evil February 17, 2008
 
2.
The act of repeatedly inserting and retracting a clenched fist at Bruce Lee speeds.

Usually into a bodily oraphis with out lube. If executed correctly uncontrollable bleeding will occur.
"I just got arrested for power fisting my girl friend, she got taken to ER"
by gazinlar November 05, 2011
 
3.
Beating someone or something senseless with a mechanical fist.
Oh man, I just got done power fisting, I'm exhausted. I think I'll go take a nap.
by Jon X6 December 03, 2008