Does this word seem like it should be about pills or pillars? Look at those pillars in the middle, illi – is that what they are, perhaps holding up a pavilion? Is this word really billion with a b that has fallen and lost its voice? Or is there a pillow involved?
Pillow? How about a cushion? How about if the illi is two people, one in front of the other? Maybe with bags or equipment… hmm… Is that a bit of a reach? If you have a saddle or motorbike seat with a cushion behind it on which a second person can ride?
Because that’s what a pillion is: it’s a cushion on which a person can ride behind the driver of a motorcycle or the person with the reins on a horse. You just pile on and try not to look like a pillock. And that makes the circles in the p and o the front and back tires of the motorbike, perhaps – as Jim Taylor noted when he suggested this word.
Pillion can also refer to the passenger, and the pillion on the pillion can be said to be riding pillion. So where does this pile of pillions come from? Probably Irish Gaelic pillín or Scots Gaelic pillean, which come from pellis, Latin for ‘skin’ or ‘hide’ or ‘leather’. Which, incidentally, is probably also what both driver and pillion are wearing.