First the term'fist' must be understood here as 'to screw someone over'/'do someone in'.
A main fist is someone who constantly/ perpetually annoys someone/fists them. Similarly, it can also mean a person who has commited a highly appalling act of fisting (non sexual). It can be used to refer to a person or a company/coporation. It can also be used as a jokey affectionate term for close friends/family.
The term originated in the north of england.
"mainfist checkout girl just fisted me trying to charge me double for my pineapple."
andy: "ye man, main fist wont stop calling my phone and now she posted pictures of me all over campus saying I'm crap in bed."
dave:"main fisting action there, mate."