An accessory to the more commonly known "wing man," the rear gunner's job is to provide even more assistance to the lead and the wing men. The rear gunner's job is to hurl insults and other harmful comments about the physical traits or mental insecurities of anyone and everyone that tries to approach either of his buddies. The rear gunner is a necessity for serious pick up situations, a job to be entrusted to a true friend and serious shit talker.
A very brave bloke who sat for hours in the rear gun turret of a WW2 bomber having the shit shot out of him by enemy fighters. It was lonely freezingly cold job with a high mortality rate.
A WW2 bomber had several defensive gun turrets, the rear one was the most vunerable to being hit by an enemy fighter's machine guns.
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.