4 Answers

When the outnumbered English faced the French at the Battle of Agincourt, they were armed with a relatively new weapon, the longbow.  The French were so amused that they vowed to cut off the middle finger of every British archer,  When the longbows won the day, the English jeered the retreating French by raising that middle finger in a  gesture that still means, among other things, "in your face."

Yvonne57

It's been seen in ancient Roman writings which is dated way before the French at the Battle of Agincourt.

maybe it wasn' a soldier who lost his four other digets in battle

from wikipedia: 

It has been identified in ancient Roman writings and reference is made to using the finger in ancient Greek comedy to insult another person.  The widespread usage of the finger in many cultures is likely due to the geographical influence of the Roman Empire and Greco-Roman civilization.  Another possible origin of the gesture can be found in the first century Mediterranean world, where extending the middle finger was one of many methods used to divert the ever present threat of the evil-eye offense.

Italy, it means to shove it up your arse.