entre nous
adverb [ after verb ], adjective formal uk/ˌɒn.trə ˈnuː/ us/ˌɑːn.trə ˈnuː/
你知我知（地），不可外传（地）；仅限于我们两人之间（地）
used when telling someone something that is secret and should not be told to anyone else
He told me - and this is strictly entre nous - that he's going to ask Ruth to marry him. 他告诉我——绝对不要跟别人说——他要向露丝求婚。