world's smallest violin?

Whenever my mom starts waxing poetic about her childhood (“And I had to slog my way through a alligator filled-bog every day just to get to school, and since my parents were too poor to buy me shoes I had to wrap my feet in wax paper, yadda yadda deeda”), I find that the quickest way to shut her up is to show her the gesture whereby I rub my thumb against my middle finger.

I only kind of know why this is a good thing to do when people are complaining about something, or looking for sympathy.

When I rub said appendages, I know that I am “playing the world’s smallest violin.” But what am i playing? A regular, run-of-the-mill song? A dirge? Or is it something specific?

And why is that an appropriate gesture for such a situation as outlined above? Should I have to play this violin for people unfamiliar with the gesture, what, exactly, should I say I am doing (“Here, I’m playing you a song on the world’s smallest violin.”?), and WHY am I doing it?

Whereforth does all this violin nonsense come from? I want to be able to insult people in an educated manner.

I’ve wondered about that too, here in England many people add ‘Here’s the world’s smallest violin playing My Heart Bleeds For You’