The Parthians lived in what is now northern Iran. They were, for about two centuries until AD 200, troublesome neighbours of the Roman Empire.
When the Parthians went into battle they had one tactic which the Romans found particularly dangerous. The Parthians were famous for their mounted archers. They specialised in sweeping manoeuvres at full gallop.
When the Romans proved too powerful, the Parthians would retreat. Just as they reached extreme range, the Parthians would turn in their saddles and shoot a final arrow while their horses galloped away. This tactic enabled the Parthians to get the last shot in any battle and became known as a Parthian shot.