There are more than one story about this.
Prior to the 15th century, from the 12th century forward, many sons of nobles went forth into battle for any number of reasons. In order to identify the bodies, the sons were sent with the crest of their family on their finger. As an insult the enemy would cut the finger with the ring so the body would not be identified or claimed.
To show they were very much alive, they would flip the ringed finger to the enemy as a matter of defiance showing their family was against them.
I heard this one before the bow explanation. I liked the bowman explanation better.
