A boy showing the middle finger to a girl generally means an obscene gesture expressing contempt or disrespect. It is roughly equivalent to telling someone "fuck you" or "get lost." This gesture is often used to insult or show anger, but it can sometimes be used playfully or humorously depending on the context and the relationship between the people involved. The gesture has ancient roots, symbolizing the phallus, and has retained its offensive meaning in many cultures, especially in the Western world. However, the intent varies; if done smiling or playfully, it might be teasing or joking rather than serious offense.

In summary, the middle finger is a rude, offensive hand sign traditionally used to show strong negative emotions or insult, but context is key to interpreting its exact meaning in a social setting.