isnt that where the middle finger came from? when archers were taken prisoner their middle fingers would be cut off so they could no longer draw a bow, so at the beginning of a battle each side shoots the bird
this IS one theory, but there are a couple others. It's not something written in stone, but that theory^ (or something similar to it) is what makes the most sense to me.
I learned about it in my European history (1200-present) class... that's why i act like i know but show no evidence, not even unreliable evidence... I'm sure the different theories are somewhere online though. I'm just too lazy.
Although... you can't always trust a professor...