Origin and history of confidence

c. 1400, "assurance or belief in the good will, veracity, etc. of another," from Old 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 assimilated form of com, here perhaps an intensive prefix (see com-), + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade").

From mid-15c. as "reliance on one's own powers, resources, or circumstances, self-assurance." The meaning "certainty of a proposition or assertion, sureness with regard to a fact" is from 1550s. The meaning "a secret, a private communication" is from 1590s.

The connection with swindling (compare con (adj.)) dates to mid-19c. from the notion of the false "trustworthiness" which is the key to the game.