between you and me and the bedpost/gatepost/four walls/lamppost In strictest confidence.

This elaboration of just between you and me is often followed by gossip about someone else. The bedpost version dates from the early nine- teenth century and was used by Edward Bulwer-Lytton (Eugene Aram, 1832: “Between you and me and the bedpost, young master has quarrelled with old master”), Dickens, and others. The lamppost version may be a little older, but is not much heard anymore.