This can be used in a couple of different ways, but typically refers to someone who is always hungry or extremely greedy and is always eating. The expression originated with the Royal Navy where the slang name for food is scran. It can also be used to refer to a person who is dirty, untidy, unkempt or of unprepossessing appearance, this too originated with the Royal Navy, perhaps because people who were always eating changed shape so that their uniforms did not fit or were so intent on food that they neglected their appearance.
“If eating was an Olympic sport, you’d be a gold medallist you scran bag!”
“Look at that scruffy sod! What a scran bag!”
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.