The source of it being 'something do with the french' was that the French king during one of the wars with the English decreed that any of the dreaded longbow archers who were captured would have those two fingers amputated such was the fear of their abilities (a longbow would go straight through heavy armour). The longbow men used to wave those two fingers at the massed French lines while shouting 'come and get them!'
In modern day UK the fingers reversed (palm forward) is a copy of the Churchillian 'Victory' sign but facing the other way it's a mixture of 'come here if you think you're hard enough' and 'f off'.
