It's like " go take a long walk on a short pier". It would be very hard to piss up a rope. It's also like telling someone to "fuck off" or "go get lost". A widely used term in the 50's and 60's by men who worked in factories together for extended periods of time.
Shakey was so mad at Ralph that he told him to "go piss up a rope". Ralph was so stupid that he actually obtained a large piece of rope and proceeded to urinate on it while it was held in a vertical position.
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.