in strict confidence
The noun post has long been metaphorical for anything deaf, lifeless or ignorant. For instance, the following was published in The World of Thursday 8th November 1753:
The business of this letter is only to vindicate from reproach a poor inanimate being, vulgarly called a Post, which everybody knows is held in the lowest contempt, yet whose services to mankind entitle it to a very high degree of regard and veneration.
‘As stupid as a Post,’ is a phrase perpetually made use of. If we want to characterize a fool, or a man absolutely without an idea, the expression is—‘As stupid as a Post.’
But already in a religious poem written in the mid-15th century, a man thus answered his conscience’s remonstrances:
Good conscience, goo preche to þe post,
Þi councel saueriþ not my tast.
Good conscience, go preach to the post,
Thy counsel does not savour [= is not pleasing to] my taste.
Any post therefore can be relied on not to reveal a confidence.
Of course, the simple expressions between you and me, between us two, between ourselves, etc., meaning in confidence, have existed for a long time. For instance, Kyng Alisaunder, a poem composed around 1400, contains the following:
Tel me who my fadir [= father] is,
Pryvely [= secretly], bytweone thè and me!
The more elaborate forms appeared in the 19th century. The earliest is found in a letter from Alabama dated 3rd May 1821, in which the American author and newspaperwoman Anne Newport Royall (1769-1854) perhaps implied, by the use of bedpost, intimacy as of people in bed together:
What think you, Matt, of the Christian religion? Between you and I*, and the bed post, I begin to think it is all a plot of the priests.
(* The form between you and I is a hypercorrection of between you and me, that is to say, the use of an erroneous word form based on a false analogy with a correct or prestigious form.)
In The Life and Adventures of Nicholas Nickleby (1839), the English novelist Charles Dickens (1812-70) simply used post:
“I called in on my way up stairs, more than half expecting to find you here,” said Ralph, addressing his niece, and looking contemptuously at the portrait. “Is that my niece’s portrait, ma’am?”
“Yes it is, Mr. Nickleby,” said Miss La Creevy, with a very sprightly air, “and between you and me and the post, Sir, it will be a very nice portrait too, though I say it who am the painter.”
The form with gatepost is first recorded in The Hoosier School-master (1871), a novel by the American historian and novelist Edward Eggleston (1837-1902):
Mirandy! Thunder! You believed Mirandy! [...] Atwixt you and me and the gate-post, don’t you never believe nothing that Mirandy Means says.
The English author Pelham Grenville Wodehouse (1881-1975) used yet another variant in A Damsel in Distress (1919):
“Your lawyers will no doubt communicate with us in due course. And, if you take my advice,” he concluded, with another of his swift changes of manner, “you’ll get ’em to settle out of court, for, between me and you and the lamp-post, you haven’t an earthly!”
Fanciful ad hoc variations are common. In his 1891 novel, In the Roar of the Sea, the English antiquarian, novelist and Anglican priest Sabine Baring-Gould (1834-1924) wrote:
“Would you mind—just another drop?”
After his glass had been refilled, Mr. Scantlebray leaned back in his chair and said:
“It’s a wicked world, and, between you and me and the sugar dissolving at the bottom of my glass, you won’t find more rascality anywhere than in my profession.”
In The Death Maker (1926), the British adventure and thriller writer Austin James Small (1894-1929) used “between you and me and that door knob”.
In 1993, the magazine The Economist had the following:
“The Liberal Democrats are another matter: tricky targets, the Lib Dems. Between you and me and that barstool, I can’t really tell the difference between their policies and ours. Best thing is to tell the voters that a Liberal Democrat vote is a vote for a Labour government. It worked last time…”