This was a military tactic that history attributes to the Parthians while retreating from an enemy. They twisted around shooting arrows at pursuing opponents while riding without stirrups at full gallop (by necessity or faking a retreat) and made a lasting impression of horsemanship. Parthian shots have been used metaphorically since the early 1800's by the leavings of snide remarks at a departure. A parting shot (of today) may be the simplified version of a Parthian shot in politics or business - but today's parthians of social-justice may only be victims of bad attitudes or poor digestion.