from
The Collaborative International Dictionary of English v.0.48
Pillion \Pil"lion\, n. [Ir. pillin, pilliun (akin to Gael.
pillean, pillin), fr. Ir. & Gael. pill, peall, a skin or
hide, prob. fr. L. pellis. See {Pell}, n., {Fell} skin.]
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.
[1913 Webster]
His [a soldier's] shank pillion without stirrups.
--Spenser.
[1913 Webster]
2. A saddle or seat behind the driver's saddle on a
motorcycle, used by passengers.
[PJC]