The Moutza: The History

A "moutza" is the most traditional gesture of insult among Greeks which consists of extending all fingers and presenting the palm towards the to-be-insulted person. An even more offensive version is achieved by using both hands to double the gesture, smacking the palm of one hand against the back of the other, in the direction of the intended recipient. The closer the gesture is to the other man's face the more threatening it's considered.

Origin
The origin of the gesture can be traced back to the Middle Ages, particularly in the penal code of Byzantium, whereby a chained criminal was paraded around town would have his face spread with cinder. Cinder in medieval Greek was called moutzos. And, because cinder was wiped on the person's face first by collecting it in the palm and then by extending open the fingers, the gesture itself became insulting, to be known as moutza, after the name of the material applied.

Reference: Wikipedia