My quite ordinary and long suffering Websters Colligiate Dic. has a second listing as an ADVERB. Thus, my understanding is that the person riding pillion may be refered to as the pillion. Lots of words are connected but have more than one understanding. One of my fav's is 'fob' which means the both the tassle used to get the watch out of the pocket and for the pocket itself. Language is difficult but can be fun. And, yes, when I look up a word, I tend to highlight it so that it is easy to find again and others get a wry smile out of the fun when they see the highlights.