I had, however, several correspondents, who kept me au fait as to the proceedings of the dear little town.
Elizabeth Gaskell. Cranford (1851)
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it's very refreshing and charming to find someone like yourself who is not entirely—er—" he blushed, searching for a word, "not entirely au fait, shall we say, with ways at Manderley.
Daphne du Maurier. Rebecca (1938)
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He had not the placid, quiet, unworldly air of Simeon Halliday; on the contrary, a particularly wide-awake and au fait appearance, like a man who rather prides himself on knowing what he is about,
Harriet Beecher Stowe. Uncle Tom’s Cabin (1852)
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"But," said I, "you are quite au fait in these investigations. The Parisian police have done this thing often before."
Edgar Allan Poe. The Works of Edgar Allan Poe — Volume 2
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It is not in the bloodless formulas of the writers of to-day, who say: au fait (for ‘in reality’), singulièrement (for ‘in particular’), étonné (for ‘struck with amazement’), and the like, that we recapture the old speech and the true pronunciation of words,