Strong's 571 (ἄπιστος)

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that believeth not, faithless, incredible thing, infidel, unbeliever(-ing)

(actively) disbelieving, i.e. without Christian faith (specially, a heathen); (passively) untrustworthy (person), or incredible (thing)

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from G1 (Α) (as a negative particle) and G4103 (πιστός);