Pretty much the same thing as breast. It is usually used by uneducated people with speech impediments, with the exception of the habit of the modern world interfering with fluent English. Some people also use this word to mock others, such as the special girl down the street, or the 'just learning to speak' two year old. Overall, breast or breastasis are loved by millions world wide.
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.