Between you, me and the bedpost
What's the meaning of the phrase 'Between you, me and the bed-post'?
The phrase 'between you, me and the bed-post' is used when divulging a secret; something that only the speaker and the listener should hear.
What's the origin of the phrase 'Between you, me and the bed-post'?
Posts, of whatever sort, have long been used to epitomise deafness and unresponsiveness; for example, Richard Braithwaite's Solemne and Joviall Disputation, 1617, compares characters as 'like Posts can neither speake nor goe'. We retain the allusion in the idiomatic phrase 'as deaf as a post'. In the 17th century posts were also called stupid; Horace Walpole made clear widespread such use in his letters in 1753:
'As stupid as a POST,' is a phrase perpetually made use of.

The earliest version used might be expected to be the unadorned 'between you, me and the post' and that elaborations would come later. That may well be the case, but the earliest citation I have found is an example of the 'bedpost' version, in Edward Bulwer-Lytton's novel Eugene Aram, 1832:
"Between you and me and the bed-post - young master's quarrelled with old master."
See also, other phrases by Bulwer-Lytton: It was a dark and stormy night, the pen is mightier than the sword.