A polite way of saying "motherfucking asshole," as Oedipal refers to the "Oedipal Complex," in which a boy/man has feeling of romantic and/or sexual love for his mother. Oedipal, in turn, comes from "Oedipus," the character in the play by Sophocles, who grows up away from his mother and later marries her without knowing who she is and his relation to her.
Anus, of course, is that posterior anatomical orifice also known as the "asshole."
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.