This stands for 'shut the fuck up you fucking piece of shit'.
Most people use this word when they are angry or annoyed.
Girl: Omg, Stacy is having an awesome party tomorrow!!!! You have to come!!! There are going to be a lot of really cute guys there and a lot of food and we are all going to play spin the bottle and get locked up in a closet with the other person for 10 minutes!!!! Omg..... <Other girl interrupts her>
Girl #2: OMFG!!!!! JUST STFUYFPOS!!!!!! DAMN! YOU ARE SOOOOOO FUCKING ANNOYING!
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.