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, me, and the lamppost
In complete confidence between the speaker and the listener, as of a forthcoming secret or rumor. Now, this is between you, me, and the lamppost, but I'm thinking about filing for divorce. Between you, me, and the lamppost, I hear that they're going to lay off half the staff by the end of the week.