From http://autosportsltd.com/featured_twofingers.php
As the story goes, hundreds of years ago when the English and the French were doing battle, the unfortunate English longbowman who fell into the hands of the enemy often had the two fingers of his "string-pulling" hand loped off. Should he escape, this would prevent him from plying his trade as effectively. That was if he was lucky. He often lost more than two fingers. Sometimes all five, or a hand, and often his head.
The English longbowmen would taunt the French by raising the two fingers across the lines to show they still had them. Through the years this gesture has remained a very English expression. In the last scene of Le Mans shown above, Steve McQueen, as Michael Delaney, salutes Ferrari driver Erich Stahler, played by Siegfried Rauch.
Photo by Nigel Snowdon