pil·lion /ˈpɪljən/
  婦女用馬鞍,後部座位
  Pil·lion n.
  1. A panel or cushion saddle; the under pad or cushion of saddle; esp., a pad or cushion put on behind a man's saddle, on which a woman may ride.
     His [a soldier's] shank pillion without stirrups.   --Spenser.
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  pillion
       n : a seat behind the rider of a horse or motorbike etc.