The popular notion that the "Middle Finger Salute" originated with the Battle of Agincourt is a Suburban Myth, one which folklore expert Barbara Mikkelson describes as "so obviously a joke that shouldn't need any debunking." As noted in an earlier post, Aristophanes explicitly refers to the middle finger gesture in The Clouds, written almost 2000 years before the Battle of Agincourt. The Romans referred to it as digitus impudicus.
In fact, many cultures have variations of "giving the finger," ranging from the upraised middle finger, to the upraised arm, to the Renaissance thumb gesture mentioned in Act I, Scene i of Romeo and Juliet. Aristophanes explicitly refers to the middle finger gesture in The Clouds, written almost 2000 years before the Battle of Agincourt. Some cultures even prefer what to Westerners would be considered a thumb's up. It is vitally important not to hitch-hike, at least western-style, in such places.
A more probable, if unprovable, theory, relates the gesture to primate behaviour. When one monkey or chimp, say, wants to establish dominance over another, it will mount it from behind in an act of what can only be termed mock rape; penetration does not actually occur. If the other primate accepts its submissive place in the hierarchy, it will tolerate this situation, briefly. The gender of the monkey assuming the "masculine" position and the monkey assuming the "feminine" one is irrelevant.
Humans share most of our genetic material with chimpanzees and bonobos. But mock rape of this nature would be rather inconvenient in human societies. Instead, we find a symbolic substitute.
The upraised finger, then, becomes a symbolic phallus. This might also account for why fuck you* and its equivalents in many other languages are an insult. It could account for why the most macho of men will, despite their frequent inclination towards homophobia, make what amounts to a request for sexual favours when they want to insult another man. The primate connection also might explain bowing, certain positions of prayer and obeisance, and the classic spanking position. Even the evil practice of actual rape during war, in prisons, in gangs to show dominance may be a human perversion of a primate ritual.
I realize this may have you wondering, "did I just symbolically rape that idiot who cut me off?" next time you flip "the bird" in traffic, but somewhere, deep within your primate brain, you just may have.
*"Fuck," of course, has a well-documented history which has been noded elsewhere, and has nothing to do with the Hundred Years' War, either.
Corbeill, Anthony. Nature Embodied: Gesture in Ancient Rome. 2003, Princeton University Press.
“F-ck.” Online Etymological Dictionary.
Laërtius, Diogenes. Lives of Eminent Philosophers VI.2.34.
Mikkelson, Barbara. "Pluck Yew." Urban Legends Reference Page.
Sagan, Carl and Ann Druyan. In the Shadows of Forgotten Ancestors. New York: Random House, 1992.