Definition of "confidential"

[…] I have a communication of a very private—indeed, I will say, of a sacredly confidential nature, which I desire to make to you.

1872, George Eliot, Middlemarch‎, Edinburgh: William Blackwood, Book 6, Chapter 61, p. 355

It would tell against your reputation, losing a confidential document, wouldn’t it? Why didn’t you keep it confidential if it was confidential?

1960, Muriel Spark, chapter 10, in The Bachelors‎, Philadelphia: Lippincott, published 1961, page 163

Long, long would it be ere Miss Crawford’s name passed his lips again, or she could hope for a renewal of such confidential intercourse as had been.

1814 July, [Jane Austen], chapter XVI, in Mansfield Park: […], volume III, London: […] T[homas] Egerton, […], page 310

I was only alive to the condensed confidential comfortableness of sharing a pipe and a blanket with a real friend.

1851 November 14, Herman Melville, chapter 11, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, page 60

She and Bertha had never been on confidential terms, but at such a crisis the barriers of reserve must surely fall:

1905, Edith Wharton, The House of Mirth‎, New York: Scribner, Book 2, Chapter 2, p. 329

Miss Raste was encouraged to be entirely confidential, to withhold nothing even about herself, by the confidence-inspiring and kindly aspect of Elsie’s face.

1923, Arnold Bennett, Riceyman Steps‎, London: Cassell, Part 5, Chapter 2, p. 241

Now, they want me to send up a confidential person with some writings.

1819, Jedadiah Cleishbotham [pseudonym; Walter Scott], Tales of My Landlord, Third Series. […], volumes (please specify |volume=I to IV), Edinburgh: […] [James Ballantyne and Co.] for Archibald Constable and Co.; London: Longman, Hurst, Rees, Orme, and Brown, […]; Hurst, Robinson, and Co. […], page 168

This paper will serve instead of a confidential friend into whose ear I might pour forth the overflowings of my heart.

1848, Anne Brontë, chapter 18, in The Tenant of Wildfell Hall‎, volume 1, London: T.C. Newby, pages 320–321

[…] perhaps the confidential bachelor clerks in Tellson’s Bank were principally occupied with the cares of other people;

1859, Charles Dickens, chapter 3, in A Tale of Two Cities, London: Chapman and Hall, […], book I (Recalled to Life), page 11

I repeated the instruction by letter and I kept a copy of the letter witnessed by my confidential maid.

1924, Ford Madox Ford, Some Do Not ...‎, London: Duckworth, Part 2, Chapter 2, p. 245

“He said he was a confidential messenger,” shouted a man.

1959, Kurt Vonnegut, chapter 6, in The Sirens of Titan‎, New York: Dial, published 2006, page 155