What does raising one's middle finger mean?

It is certainly thousands of years old, being referred to in Ancient Roman literature as the digitus infamis or digitus impudicus. Performing this gesture is also called "flipping the bird" in countries where "the finger" is used. In other regions, "flipping the bird" refers to the raising of the middle and index finger with the back of the hand directed at the recipient. It can also mean "Victor" (see V Sign, below) in some countries, which is not to be mistaken for the "Peace" gesture, which is done with the palm facing the recipient of the gesture, but in Britain and some other countries it is an offensive gesture, equivalent to "the finger".

When this gesture is made with the palm facing forward, it is known to Chaotes practicing Lovecraftian magick as the "Sign of Kish".

http://www.search.com/reference/Type_of_gesture#Middle_finger

The "one-finger salute," or at any rate sexual gestures involving the middle finger, are thousands of years old. In Gestures: Their Origins and Distribution, Desmond Morris and colleagues note that the digitus infamis or digitus impudicus (infamous or indecent finger) is mentioned several times in the literature of ancient Rome. Turning to our vast classical library, we quickly turn up three references. Two are from the epigrammatist Martial: "Laugh loudly, Sextillus, when someone calls you a queen and put your middle finger out."

In the other reference Martial writes that a certain party "points a finger, an indecent one, at" some other people. The historian Suetonius, writing about Augustus Caesar, says the emperor "expelled [the entertainer] Pylades . . . because when a spectator started to hiss, he called the attention of the whole audience to him with an obscene movement of his middle finger." Morris also claims that the mad emperor Caligula, as an insult, would extend his middle finger for supplicants to kiss.

http://www.straightdope.com/classics/a980904.html