between you and me
Also, between ourselves; just between you and me and the bedpost or four walls or gatepost or lamppost. In strict confidence. For example, Just between you and me, it was Janet who proposed to Bill rather than vice versa. This phrase, dating from about 1300, is generally followed by some informative statement that the listener is being asked to keep secret. The variant with bedpost, also shortened to post, dates from the early 1800s; four walls, also shortened to the wall, dates from the early 1900s, as does the gatepost.