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.
Ah, I see. We don’t tend to put it to the mouth. It’s more “fuck you”. Apparently comes from demonstrating to the French that you still have your bow-drawing fingers and intend to use them. British archers captured by the french would have their first two fingers removed to prevent them launching arrows.