A arm barn is the southern term for a circle jerk. Where a group of good ole boys get together to jerk each other off in a circle. An arm barn tends to be more rough than a circle jerk on account of all participants having blue collar jobs, thus leading to hands that know a hard day's work.
After working a hard day down at the mill, Brady was looking forward to getting together with the boys for a good old arm barn later.
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.