Sentence:  "I am, as it were," she concluded, "the <WSD>chaperon</WSD> of my young cousin, Lucy, and it would be a serious thing if I put her under an obligation to people of whom we know nothing. His manner was somewhat unfortunate. I hope I acted for the best." 
Meaning of the word marked with <WSD> tags:  An adult who accompanies or supervises one or more young, unmarried men or women during social occasions, usually with the specific intent of preventing some types of social or sexual interactions or illegal behavior.