Act 1, Eliza says this when Higgins tells Pickering that he can place any man within two miles in London.
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Higgins's ability to pinpoint the location of origin of members of the crowd means not only that he can tell what part of England, or even what neighborhood of London, they are from, but also that he can probably guess fairly easily their socioeconomic status. In the early twentieth century, social mobility in Britain was slim to none. His assertion that he can turn a flower girl into a duchess is rather offensive to the flower girl.
She is criticizing how this guy feels he can place people's accents and social positions in society by listening to how they speak.