between you and me and the bedpost
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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 bedpost (or the gatepost or the wall)in strict confidence. informal
The bedpost , gatepost , or wall is seen as marking the boundary beyond which the confidence must not go.