pil ‧ lion /ˈpɪljən/ BrE AmE noun [singular] British English
[ Date: 1400-1500 ; Language: Scottish Gaelic ; Origin: pillean or Irish Gaelic pillin , from peall 'couch' ]
the seat behind the driver of a ↑ motorcycle :
a pillion passenger
—pillion adverb :
Tom had never ridden pillion before.