/ ˈpɪliən; NAmE / noun
a seat for a passenger behind the driver of a motorcycle :
a pillion passenger / seat
► pil·lion adverb :
to ride pillion
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WORD ORIGIN
late 15th cent. (denoting a light saddle): from Scottish Gaelic pillean , Irish pillín small cushion, diminutive of pell , from Latin pellis skin.