THE MIDDLE FINGER
Also known as:  the one-finger salute  the highway salute  flipping (someone) off  flipping the bird  the Trudeau salute [in Canada]
Instructions to show the gesture: Keep your middle finger raised, while bending the rest of your fingers at the second knuckle. [helps if u have long fingers, also helps if u breathe out "F*** you"]
Origin: Ze history of the middle finger! Well, the interest came into being after I found a cool ASCII sig. So, a little research brought out some interesting info that I have been updating myself with, prior to the Software Engineering Series exam. =D!
There are many versions as to how the middle finger came into being. According to one story, the "single finger salute" actually has a meaning.
It was the Battle of Agincourt in 1415 that brought about the "middle finger" being a derogatory gesture. The Frech anticipating the victory over the English threatened to cut off their middle fingers. Why the middle fingers you may ask? Well, the English Longbow was the most feared weapon of the time - the Englishmen being world renown archers. So, on chopping off the middle fingers, the archers would be unable to shoot arrows. [Note professional archers even today hold the arrow with their index and middle fingers =D]. But, the threat never materialized as the English held their ground and even won the battle. Consequently, the English started mocking the French by giving them the "single finger salute".
It gets better. As the English longbow was made with Yew, using the longbow was called "Plucking the Yew". So along with erecting the middle finger, the English also used to say "See we can still Pluck Yew!" Now, as folk etymologies go, a lot of etymologies have grown around this gesture. As it's difficult to pronounce "pluck yew", [read pleasant mother pheasant plucker - the person who you had to go to for the feathers used on the arrows for the longbow], this consonant cluster gradually changed to the labiodental fricative 'F', yielding probably one of the most popular phrases of this century. Well, another piece of news is that the arrows also used "pheasant" feathers. Hence the reason as to why this gesture is also known as "giving the bird".