The act of urinating on a woman's head, effectively drenching the hair, and then proceeding to have the woman give the man a blowjob whilst her wet hair is draped in front of her face, ergo dirty curtain.
A male with long hair lies on his back on his bed with his hair hanging to the floor. The female lays exposed on the floor and let's his hair tickle her beaver.
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.