Etymology
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confidence (n.)
Origin and meaning 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." Meaning "certainty of a proposition or assertion, sureness with regard to a fact" is from 1550s. Meaning "a secret, a private communication" is from 1590s. The connection with swindling (see con (adj.)) dates to mid-19c. and comes from the notion of the false "trustworthiness" which is the key to the game.

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self-confidence (n.)

"confidence of one's own judgment or ability, reliance on one's own powers without other aid," 1650s, a back-formation from self-confident, or else from self- + confidence.

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overconfidence (n.)

also over-confidence, "excessive confidence, state of being overconfident," 1700, from over- + confidence.

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confidential (adj.)

1759, "indicating the confiding of a private intimacy," from Latin confidentia (see confidence) + -al (1). Sense of "intended to be treated as private" is from 1773; that of "enjoying the confidence of another" is from 1805. Related: Confidentially.

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con (adj.)

"swindling," 1889 (in con man), American English, from confidence man (1849), from the many scams in which the victim is induced to hand over money as a token of confidence. Confidence with a sense of "assurance based on insufficient grounds" dates from 1590s. Con artist is attested by 1910.

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diffidence (n.)
Origin and meaning of diffidence

c. 1400, "distrust, want of confidence, doubt of the ability or disposition of others," from Latin diffidentia "mistrust, distrust, want of confidence," from diffidere "to mistrust, lack confidence," from dis- "away" (see dis-) + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade"). The opposite of confidence. Original sense (distrust of others) is obsolete; the modern sense is of "distrust of oneself, want of confidence in one's ability, worth, or fitness" (1650s), hence "retiring disposition, modest reserve."  

Diffidence is a defect: it is an undue distrust of self, with fear of being censured for failure, tending to unfit one for duty. [Century Dictionary]
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*bheidh- 
Proto-Indo-European root meaning "to trust, confide, persuade."

It forms all or part of: abide; abode; affiance; affidavit; auto-da-fe; bide; bona fide; confederate; confidant; confide; confidence; confident; defiance; defy; diffidence; diffident; faith; fealty; federal; federate; federation; fiancee; fideism; fidelity; fiducial; fiduciary; infidel; infidelity; nullifidian; perfidy; solifidian.

It is the hypothetical source of/evidence for its existence is provided by: Greek pistis "faith, confidence, honesty;" Latin fides "trust, faith, confidence, reliance, credence, belief;" Albanian be "oath," bindem "to be convinced, believe;" Old Church Slavonic beda "distress, necessity," bediti "to force, persuade;" Old English biddan "to ask, beg, pray," German bitten "to ask."
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self-assurance (n.)

"feeling of confidence and security as to oneself," 1590s, from self- + assurance.

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unbosom (v.)
"disclose in confidence" (secret opinions or feelings), 1580s, from un- (1) + bosom.
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bunco (n.)
also bunko, type of confidence swindle, 1872, perhaps from Italian banco "bank."
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