Broken down, "pissed-up" means inebriated or drunk. A "slapper" is, well, a slut. Or, a woman of loose morals. So, a "pissed up slapper" is also known as a drunken trollop or whore.
"Lets go to the bar tonight and pick up a couple of pissed up slappers." "Check that pissed up slapper going down on Mark in the corner." "It's a sure thing cause she's a pissed up slapper."
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.