Why is the rude finger rude?

In the war, the archers would shoot the advancing enemy. The enemy were threatening to cut off the archers' middle fingers if they lost. When the archers' army won, the archers stuck their finger up at the enemy as if to say 'na na nana na, we've still got our fingers!' Unfortunately, now, it means something a lot worse.* *For those of you who don't know what it means, it means ***************. (ittle kids use this site too you know!)