Confidentially

In a confidential manner; privately and secretly, with the expectation that information will not be shared with others. Spoken or done in confidence.

From Latin 'confidentia' from 'confidere' (to have full trust) combining 'con-' (with) and 'fidere' (to trust). The adverbial form developed in English by the 17th century, maintaining the core meaning of communication based on trust and secrecy.