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.