Confidence descends from Latin confīdentia, built on confīdere — literally 'to trust fully' — from con- (intensive) + fīdere (to trust), from fidēs (faith), PIE *bʰeydʰ-. The word carries a fundamental duality: inner self-assurance, trust placed in another, or — since 1849 — the mechanism of a swindle.
The feeling or belief that one can have faith in or rely on someone or something; firm trust. Also: a state of self-assurance arising from an appreciation of one's own abilities.
From Latin confīdentia (full trust, bold assurance, reliance), from confīdēns, present participle of confīdere (to trust fully, to rely upon completely), composed of con- (together, intensive, fully) + fīdere (to trust), from fidēs (faith, trust, reliability), from PIE *bʰeydʰ- (to trust, to persuade). The PIE root *bʰeydʰ- is the ancestor of the Germanic faith-words: Old English bīdan (to wait for, to trust in) gives English bide and abide; the same root gives Latin fīdēs (faith), which generates the entire fid-/fid- family: fidelity, confide, diffident (lacking trust), fiancé (one to whom trust is pledged), affidavit (he has pledged faith), and the word faith itself through Old French. The intensive prefix con- (here fully or completely) heightens trust
'Con man' is short for 'confidence man' — a term first printed in the New York Herald in 1849, describing swindler William Thompson who'd ask strangers: 'Have you confidence in me to trust me with your watch until tomorrow?' Those who obliged never saw him again. The word built on Latin's highest civic virtue — fidēs, the sacred bond — became the name