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In Western culture, the finger or the middle finger (as in giving someone the (middle) finger or the bird or flipping someone off) is an obscene hand gesture. The gesture communicates moderate to extreme contempt, and is roughly equivalent in meaning to "fuck off," "fuck you," "shove it up your ass," "up yours," or "go fuck yourself." It is performed by showing the back of a hand that has only the middle finger extended upwards, though in some locales, the thumb is extended. Extending the finger is considered a symbol of contempt in several cultures, especially in the West. Many cultures use similar gestures to display their disrespect, although others use it to express pointing without intentional disrespect toward other cultures. The gesture is usually used to express contempt but can also be used humorously or playfully.
The gesture dates back to Ancient Greece and it was also used in Ancient Rome. Historically, it represented the phallus. In some modern cultures, it has gained increasing recognition as a sign of disrespect, and has been used by music artists (notably more common among hardcore punk bands and rappers), actors, celebrities, athletes, and politicians. Most still view the gesture as obscene. The index finger and ring finger besides the middle finger in more contemporary periods has been likened to represent the testicles.

Source: Wikipedia