fid

The root fid comes from Latin fidēs (faith, trust, confidence) and its verb fīdere (to trust). To a Roman, fidēs was the glue of society: the trust you placed in a person, a promise, a god. Almost every English word built on this root is really asking one question — how much trust is on the table, and which way does it flow?

The prefix tells you the answer.

- con- (thoroughly) + fidēs → trust completely. From this single combination grew a whole cluster. Confidence is the noun for trusting fully — and notice it points two directions: trust in yourself (self-confidence) and trust in someone else ("I told her in confidence"). Confident is the adjective. To confide in someone is to hand over your trust, which is why it means to share secrets. The person you hand them to is your confidant. And something confidential is information held in trust — meant only for trusted eyes.

- fidēs alone, with the noun suffix, gives fidelity — faithfulness, staying true. Engineers borrowed it too: "high fidelity" means a recording stays faithful to the original sound.

- dif-/dis- (apart, not) + fīdere → trust pulled apart. Diffident literally means "trusting away from" — but the trust that's missing is trust in oneself. A diffident person is shy and hesitant, lacking self-confidence. (Don't confuse it with "indifferent"; that's a different word.)

- per- (through, to destruction) + fidēs → faith broken right through. Perfidy is deliberate betrayal of trust; perfidious describes the traitor.

- af- (toward) + fīdere → the Latin phrase affidāvit, "he has pledged his faith." An affidavit is a sworn written statement — you stake your trustworthiness on its truth.

Three members reached English through Old French rather than straight from Latin, so they wear a different coat — f-a-i- instead of f-i-d-. Latin fidēs became French feid/fei, then English faith; from it come faithful and faithfulness. Fealty (the loyalty a vassal owes a lord) is the same word that took a more medieval path. And defy is the oddest cousin: Old French desfier = des- (un-, away) + fier (to trust, from fīdere). To defy someone was originally to renounce your bond of faith with them — to declare you no longer owe them loyalty. That renunciation hardened into the modern sense: openly resist, refuse to obey. Defiant keeps the same spirit.

So the whole family is a map of trust: give it fully (confide, confidence), keep it (fidelity, faithful, fealty), lack it in yourself (diffident), or break it (perfidy, defy).