While technically a legal principle, this phrase actually represents a moral standard within human conscience by which we judge what others say.

falsus en uno, falsus en omnibus (fal·sus en u·no fal·sus en om·ni·bus): / FAHL suhs ehn OO no FAHL sus ehn OM nuh buhs / (always italicized)
phrase

(1) a legal brocard (an elementary maxim expressed in short proverbial form) based on Roman law by which a witness is found to be not credible in all matters if it is discovered some portion of their testimony has been willfully falsified; the premise discredits the witness’ testimony in toto if it cannot be corroborated
(2) the philosophical/metaphysical concept of human nature whereby one bases trustworthiness on cumulative and consecutive judgments as to the validity of what is said; one of the basic meanings of idioms such as “hurt me once, shame on you; hurt me twice, shame on me” and “once bitten, twice shy”

[From Latin falsus en uno, falsus en omnibus meaning “false in one thing, false in everything.”]

Usage: Juries inherently and subconsciously apply falsus en uno, falsus en omnibus when a witness is shown to be perjurious, so they therefore apply a more critical eye to all of the witness’ testimony while generally considering it untrustworthy.