Why Do We Stick Two Fingers Up To Tell F*** Off?

This actually dates back to the so-called 'Hundred Years War'. This took place between England and France from 1337 to 1453.

According to a popular myth that is, not surprisingly popular with the British, the 'two-fingered salute' originated from the English longbowmen. If bowmen were captured by the French, they often had their middle fingers cut off. This would obviously end their usefulness as a bowman.

The myth is that the English bowman would give the two-finger gesture to the French before commencing battle. It signified that they still had all their fingers in tact and that they were preparing to attack the French with their bows.

English bowman were particularly efficient and were responsible for a large number of French knights' deaths during the Hundred Years War. This sign of defiance was also meant to act as a warning and strike fear into the French, rather than the insult it is today.

The answer to this goes back to the Middle Ages and to a time when England was at war with France. It was a time of Agincourt and Crecy.
The English archers were formidable. They could send up a devastating curtain of arrows releasing the next arrow before the first had hit the ground. The English longbow was a much better weapon in many respects than the crossbow. It was simple, powerful, easy to maintain, easy to replace, easy to carry. A hail of arrows would descend on the opposing army before they could strike a blow, and then the infantry and cavalry could move in to mop up the remainder.
In one battle the French were triumphant. They captured a lot of English bowmen and cut off their index and second fingers so they would never be able to draw a bow string again.
In the next battle the English were victorious, and as the French prisoners were led away, all the English bowmen held up their two forefingers to them.