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in confidence

noun

full trust; belief in the powers, trustworthiness, or reliability of a person or thing: We have every confidence in their ability to succeed.
belief in oneself and one’s powers or abilities; self-confidence; self-reliance; assurance: His lack of confidence defeated him.
certitude; assurance: He described the situation with such confidence that the audience believed him completely.
a secret that is confided or imparted trustfully: The friends exchanged many confidences over the years.
(especially in European politics) the wish to retain an incumbent government in office, as shown by a vote in a particular issue: a vote of confidence.
presumption; impudence: Her disdainful look crushed the confidence of the brash young man.
Archaic. something that gives confidence; ground of trust.

Idioms

in confidence, as a secret or private matter, not to be divulged or communicated to others; with belief in a person’s sense of discretion: I told him in confidence.

noun

a feeling of trust in a person or thingI have confidence in his abilities
belief in one’s own abilities; self-assurance
trust or a trustful relationshiptake me into your confidence
something confided or entrusted; secret
in confidence as a secret

early 15c., from Middle French confidence or directly from Latin confidentia, from confidentem (nominative confidens) “firmly trusting, bold,” present participle of confidere “to have full trust or reliance,” from com-, intensive prefix (see com-), + fidere “to trust” (see faith). For sense of “swindle” see con (adj.).

Also, in strict confidence. Privately, on condition that what is said will not be revealed. For example, The doctor told her in confidence that her mother was terminally ill, or He told us in strict confidence that Gail was pregnant. This idiom was first recorded in 1632. Also see take into one’s confidence.

In addition to the idiom beginning with confidence

confidence game

also see:

in confidence
take into one’s confidence