The background to this phrase starts with a man named 'Chopper Read' who although being a notorious and accomplished criminal himself in Australia decided to take it upon himself to eradicate a fellow criminal from this world for an act he comitted which has now been coined Jam Jarring. It goes that the murdered man was in prison for raping a 10 year old boy. He was then released from prison slightly earlier for his good behaviour, not long after his release he had approached a 9 year old girl in a park, taken her into a quieter area of the park, he then broke a jam jar in half using the jagged ends he cut a whole in the leg of the girl and proceded to abuse this area of her body even after she had died.
When he returned to prison Chopper was on library duty and as he walked into the other man's cell he pushed him to the floor and jumped on his head from the bed until he died.
The term Jam Jarring is now employed as a means of conveying the notion of desire to follow through with this act on someone else.
Guy 1: "She is so hot I would definitley Jam Jar her."
Guy 2: "Dude that is so nasty WTF?!"