Wikipedia
A mountza (μούντζα ) (or moutza (μούτζα )) or faskeloma (φασκέλωμα ) is the most traditional gesture of insult among Greeks: it consists of extending all fingers of one or both hands and presenting the palm or palms towards the to-be-insulted person in a forward motion.
It is often coupled with the expletives να (na, 'here') and παρ'τα (par'ta, 'take these'). The closer the gesture is to the other person's face, the more threatening it's considered.
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.
If a Greek wants to hand-signal the number 5 to someone, they take care not to overextend the fingers, or to face their palm towards themselves and the back of the hand towards the intended recipient of the signal, lest it be mistaken for a moutza.