between you, (and) me, and the gatepost

What is going to be or has been said should not be told to anyone else; between us. This phrase is usually said along with information that needs to be kept secret. Primarily heard in UK. I overheard the boss talking to her secretary last night and—just between you, me, and the gatepost—she's giving the promotion to George after all. Between you and me and the gatepost, Stephanie is not as qualified for this job as she claims to be. Now, this is between you, me, and the gatepost, but I'm thinking about filing for divorce.

between you, me and the ˈgatepost

(British English, informal) used to show that what you are going to say next is a secret: Well, between you, me and the gatepost, I heard that she’s pregnant.