What do you mean by "your old granny can sit on a pin"? It seems an insulting word, but I don't know what it means.
3 little pigs.
ex) I huffed. And I snuffed. And I sneezed once again. Then the third little pig yelled. "And your old granny can sit on a pin!"
Now I'm usually a pretty calm fellow. But when somebody talks about my granny like that, I go a little crazy.
It's just a fancy mild insult. A ruder way to say this would be, for example, "You can go hang yourself."
Yes, exactly like this, but mild. "Go die" would be very inappropriate in a children's story. It's very strong language. "Your old granny can sit on a pin" is just funny. Perhaps the world would be a better place if we solved our problems by playing silly pranks (like placing a pin on someone's chair) on one another instead of waging wars.
Don't look for logic in this expression. It's supposed to be funny and expressive, not logical.