between you me and the bedpost
between you, (and) me, and the bedpost
What is going to be or has been said should not be told to anyone else. This phrase is usually said along with information that needs to be kept secret. I overheard the boss talking to her secretary last night and—just between you, me, and the bedpost—she's giving the promotion to George after all. Between you and me and the bedpost, Stephanie is not as qualified for this job as she claims to be.
between you (and) me and the bedpostand 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.