As well as Nancy being a thief, a common suggestion is that she is a prostitute, in the modern sense of the word. At no point is this stated directly in the novel; rather it stems from Dickens describing her as such in his preface to the 1841 edition ("the boys are pickpockets, and the girl is a prostitute"). However, it has been speculated that he is invoking the term's then-synonymous usage referring to a woman living out of wedlock or otherwise on the margins of "respectable" society.
