A pillion is a seat behind the rider for a passenger. Ride pillion, a pillion passenger, sit on the pillion.
Riding pillion is the fixed phrase, and it means on the back of a motorcycle without any control over what happens. The word is Gaelic and originally named a light saddle or pad on a horse.
1nounpillionA seat behind the rider for a passenger.
“She rode pillion on her brother's motorbike, holding tightly to his waist.”
pillion is a seat.
Scottish Gaelic pillean means a little rug or pad, from Latin pellis, a hide.
Latin
pellis — animal skin, pelt
Scottish Gaelic
pillean — little rug
English, today
pillion
From Scottish Gaelic pillean (“little rug”), from Latin pellis (“animal skin, pelt”).
Etymology from Wiktionary, CC BY-SA