According to Wikipedia, it originated in ancient Greece and has always meant the same thing as it does now.

That's what I heard, but the fellah who told us both might have been making it up

That's middle and index fingers, with the palm facing backwards. Just a backwards peace sign.

In the UK two fingers up is a rude gesture and it comes from battles with the french. If they caught a British archer they removed those fingers so they couldn't fire a bow. So sticking them up at the enemy and gesturing was showing they had them and would use them to fire arrows at them. I am not an historian, though, and this could just be one of those tales that sounds so true everyone believes it and passes it on.