A parting shot or Parthian shot is a final cutting remark or severe glance of disapproval, given by one person to another as they part an argument or meeting. The timing is such that it leaves one’s opponent or rival no time in which to reply or react.
The phrase is derived from Western Asia; the favourite tactics of the ancient Parthians were to discharge missiles and arrows backwards whenever their horsemen and soldiers were retreating from the enemy, hence, ‘Parthian shot’, or shaft.
P.S: if someone does this realise the immaturity of the act. So compassion 🙂