1
general::
Phrase(s): between you (and) me and the bedpost [and] between you and me and these four walls
Fig. a somewhat affected way of signaling that you are about to tell a secret. • Alan: What’s wrong with Ellen these days? She seems so touchy. Jane: Between you and me and the bedpost, I’ve heard that her boyfriend is seeing someone else. • Jill: How much did you get for your used car? Jane: Well— between you and me and these four walls—five thousand dollars.
McGrawhill's American Idioms And Phrasal Verbs