entre nous
/ˌɒ̃trə ˈnuː
/adverb
Between ourselves; privately: entre nous, the old man’s a bit of a case
Synonyms
between ourselves, between us, between you and me, in (strict) confidence, confidentially, in private, privately, off the record
informal between you and me and the bedpost/gatepost/wall
Latin sub rosa
archaic under the rose
Origin
French.