Por lo que he oido es por una batalla con los franceses, en los que los ultimos cortaban los dedos indice y corazon de los arqueros para que no pudieran disparar, y estos al vencer les mostraron los dedos, como para decir aqui los tengo todavia.
de la wikipedia:
According to a popular legend, the two-fingered salute or V sign derives from the gestures of longbowmen fighting in the English army at the Battle of Agincourt (1415) during the Hundred Years' War. According to the story, the French claimed that they would cut off the arrow-shooting fingers of all the English and Welsh longbowmen after they had won the battle at Agincourt. But the English came out victorious and showed off their two fingers, still intact. Historian Juliet Barker quotes Jean Le Fevre (who fought on the English side at Agincourt) as saying that Henry V included a reference to the French cutting off longbowmen's fingers in his pre-battle speech. If this is correct it confirms that the story was around at the time of Agincourt, although it does not necessarily mean that the French practised it, just that Henry found it useful for propaganda, and it does not show that the two-fingered salute is derived from the hypothetical behaviour of English archers at that battle.