My little contribution. When the french (they don't deserve capitalization) captured an English archer, they would cut off the left thumb. That way he couldn't hold a bow.
You really only need a thumb and finger on the other hand to hold the arrow when you nock it. Middle eastern and asian archers frequently used a thumb ring to grasp the bowstring and draw the bow. In fact, this allowed them to pull a greater bow weight. Since the 100 years war occurred well after the Crusades, they were familiar with the technique and I'm sure that an archer who had a finger amputated, making it tough to draw in the European style, would have found out about the eastern style and used it.
As to the upright middle finger, I think the explanation is pretty simple. The upright finger and knuckles on either side is a fairly good visual representation of an erect penis and testicles, hence f**k you.