The middle finger is associated with the penis and showing it to someone is meant as a sign of disrespect. The practice dates back thousands of years.
The gesture is completed by curling the other fingers around at either side to represent the testicles. It is also known as "flipping the bird." The sound of birds has long been associated with taunting, and the term came into use in the 1960s.
The connection between the middle finger and phallus is first mentioned in Aristophanes play "The Clouds," and the first known example of showing the middle finger as disrespect comes from Green philosopher Diogenes to an orator named Demosthenes in the fourth century B.C.