Middle Finger

What are the origins of the middle finger (when soley upright) having such an impolite meaning?

Here is the snopes link that addresses a common legend on this subject. Look at it before you read any of the other replies to this thread.:

Snopes on the middle finger.

Um, you might want to check out this thread in GQ, just two threads below this one, on the same subject!

In that thread is a link to Cecil’s answer, and several other good links.