Flip The Bird
This idiom is a non-vulger way to describe the middle finger solute. Many people will 'flip the bird' in traffic when cut off. I am not sure of the origins.

This from a website visitor:
I have heard that this term comes from a practice during the middle ages of cutting off the bow finger of the enemy to disable the armies. The bow finger was the middle finger. In defiance, the warriors would hold up the middle finger to show they were still capable of attacking. (来源：英语麦当劳－英语学习门户 http://www.EnglishCN.com)