Why did Oscar Wilde call Ada Leverson 'Sphinx'?

Oscar Wilde, famous for having written such plays as The Importance of Being Earnest and his only novel, The Picture of Dorian Gray, was a close friend of Ada Leverson, another writer. According to such sources as Wikipedia and 'Wilde', a 1997 movie based on a biography of Wilde's life, he called... show more Oscar Wilde, famous for having written such plays as The Importance of Being Earnest and his only novel, The Picture of Dorian Gray, was a close friend of Ada Leverson, another writer. According to such sources as Wikipedia and 'Wilde', a 1997 movie based on a biography of Wilde's life, he called Leverson by the nickname Sphinx. Does anyone know why?
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