You're a mean one, Mr. Grinch!
You really are a heel.
You're as cuddly as a cactus,
You're as charming as an eel,
Mister Griiiiinch!
You're a really bad banana
Meaning that even though you already have a girlfriend doesn't mean that you can't date someone else.
Bob: Dude look at those hotties man!
Mike: But I already have a girlfriend Bob: Even though you're on a diet, doesn't mean that you can't look at the menu.
Mike: Oh OK then.
Meaning that if you already have a girlfriend/lover, doesn't mean that you can't check out other girls.
Guy #1: Hey look at those chicks over there!
Guy #2: Buddy, I already have a girlfriend.
Guy #1: Just because you're on a diet, doesn't mean that you can't look at the menu.
Guy #2: I suppose you're right. *Gazes at girls for 5 minutes* Damn! This is like Baywatch!
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.