My Grandmother used this phrase whenever she didn't want us to know what something was.
Example: What am I getting for Christmas? Her answer was "Cat's fur for kitten's britches." meaning none of your business. Grandmom was born in 1905 so it goes back at least that far.
A female member of a co-ed sports team that is not particularly skilled, but is necessary to fulfill the minimum required number of females on the field so the team will not have to forfeit the match.
"Henrietta hasn't played soccer before, she's the 'forfeit kitten' today...but who cares if she can't play, at least she looks damn hot in those knee socks and shin guards..."
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.