Sentence:  It was all the greater triumph to Miss Nancy Lammeter's beauty that she looked thoroughly bewitching in that costume, as, seated on the <WSD>pillion</WSD> behind her tall, erect father, she held one arm round him, and looked down, with open-eyed anxiety, at the treacherous snow-covered pools and puddles, which sent up formidable splashings of mud under the stamp of Dobbin's foot. 
Meaning of the word marked with <WSD> tags:  A pad behind the saddle of a horse for a second rider.