actually a lesser known origin of the finger, which probably made it popular, is in the 14th and 15th century when France and england were at war, the French, when they captured an English archer, used to cut off their middle fingers so they couldn't use their bow anymore, so whenever a Englishman passed by a Frenchman, the middle finger was shown as to say, "look, I still have my middle finger, you stupid SOB" or something along those lines.