Sentence:  The king's cavalry are also trained to an exercise called the "keykaj", which consists in turning about on the saddle at full speed and firing a carbine backward. This they learn from their childhood, and it gives them great confidence and dexterity on horseback. It is probably a remnant of the old <WSD>Parthian</WSD> custom so frequently alluded to in ancient authors; with this difference, that fire-arms are now used instead of bows and arrows. 
Meaning of the word marked with <WSD> tags:  Relating to Parthia or Parthians.