An internet style response for a period of interactive amusement followed by an abrupt realization that you are no longer amused. Legend has it that Queen Victoria uttered the phrase right before her much more famous phrase, "We are not amused."
It could mean that you were laughing along with a joke and didn't get it, or when you finally did get it, you didn't like it because you realized that you were the butt of the joke, or it could be when you have just realized that the speaker was not joking.
Guy: "Baby, you're beautiful like a sunset, magnificent like the rainbow, you make me want to lasso the moon and give it to you, and now I want to steal you away to my desert home and make you my fourth slave-wife."
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.