It means "up yours!". The index finger and middle finger form a "V". The palm is facing inward.

This gesture is often misunderstood by foreigners who see it as the V for Victory sign. The palm facing inward is the crucial difference, which changes the whole meaning of the gesture.

This originates from the time when British bowmen were an extremely potent military force. When these archers were captured, their great enemies, the French, would sever these two fingers so that they could no longer pull the bowstring, thus literally disarming them. When the English defeated the French at Agincourt with the help of their expert archers, the English insulted the retreating French with the "V" sign, palm inward, to show both fingers intact.