There are a couple of theories about the origin of this display, though it's meaning is pretty much an accepted fact. My favorite is the following, but I can see how a longer history would not conflict with the following account.
Origin of The Finger
(This may or may not be true but it is told repeatedly....so here you go)
In 1415, Henry the V took his army of around 6,000 men across the English Channel and into southern France. After cutting trees and preparing large pointed sticks, Henry marched his army northwest for 17 days and over 270 miles. With only one day's rest, the mighty force was haggard and exhausted by the time they reached the flat land between the forest of Agincourt.
The French army, consisting of 25,000 troops, 15,000 of which were mounted knights in armor, arrived on the evening of October 24. Their army was a mishmash of Frenchmen from all over the feudal country. It rained hard that night, and both armies were soaked to the bone my morning. Most French knights slept in the saddle so as not to sully their expensive and ornate armor.
On the morning of October 25, the French and British armies were salty and ready to fight. Henry moved his troops slowly up the 1/2 mile wide passage between the two forests of Agincourt. By 11 Am, the French commanders were still bickering over tactics and whether or not to charge all the while the British were within 400 yards of the French.
Now what made the battle of Agincourt so interesting was the introduction of the Welsh long bow. This weapon could dismount a rider at 300 yards and with top notch arrows, could pierce armor at close range.
The French King had heard of the longbow and smugly claimed that when the fight was over, he would cut the bow finger from every Englishman in France. As the long bow was made from the yew tree, it took great strength to draw the bow back and without the middle finger, this would be impossible.
When Henry had his troops within bowshot of the French, he loosed the first of many volleys of arrows. The French, caught off guard, charged with half their forces. The Duke of Orleans barely made it 200 yards before his knights broke and ran under a hail of deadly wood and steel. Many of the knights sank into the mud and were trampled as the horses and frightened soldiers pummeled them into a fine paste.
Those knights that did make it to the British front lines were let upon by unarmored soldiers carrying short swords, who plunged their blades into the joints of the French armor. All this came after a great number of horses were impaled upon the huge pointed sticks the British had placed in the ground in front of them.
At the end of the day, the French had lost some 10,000 men, and the British mourned only 500 dead. In one day, the Hundred Years War had turned and the long bow had successfully defeated the myth of the invincible knight in armor.
The French nobility stood horrified on a hill over looking the battle when the Englishmen in mass turned to face them, middle finger held high for the French to see.
Shakespeare went on to glorify this battle, and the French, to vilify it. For the next 100 years, every lad over the age of 6 in Britain was required to be instructed in the firing and maintaining of the long bow.
In response to this, the French began cutting off the index and middle fingers of all British men caught in battle or on French land, thus removing the digits that allowed the firing of a bow. This is where the British tradition of waving two fingers at someone as an insult arose. And, the very American middle fingered salute or "the bird," is a descendant of this. As the feathers on the arrow were made of pheasant feathers, the saying “Giving the bird” soon arose.
As the Brothers C are of Greek descent (yes, our ancestors were formulating representative democracy whilst yours were buggering their mothers, eating each other, and living in caves) this account also has a certain appeal, though as a redneck I have to say I prefer the former but will accept the combination of the two. Ignore the Freudian bullshit:
Giving someone "the finger" is one of the basest violations in modern culture, but its origins date back over 2500 years. The first written record of the insult occurred in ancient Greece, where the playwright Aristophanes (the Adam Sandler of his day) made a crude joke mixing up the middle finger and the penis. Even back then, the bird was considered an aggressive, phallic put-down.
It has been argued by anthropologists that the finger is a a variant of a classic "phallic aggressive" gesture used by primates. By jabbing a threatening phallus at your enemy like a wild animal, you aren't just belittling him, but also making him your sexual inferior. Instead of using a real penis, civilized Janes and Platos called upon the substitute wieners within their own hands to mock, threaten, and humiliate opponents.
And boy, did it. When the Romans imported the art, music, and culture of the Greeks, the finger came along, too. Roman Emperor Caligula, a pioneer in perversity, frequently shocked his citizens by forcing them to kiss his middle finger instead of his hand. One of his subjects, Cassius, who Caligula often taunted as being too effeminate, finally had enough humiliation and assassinated him. Clearly, the bird was not to be taken lightly.