/ ˈɪnfɪdəl; NAmE / noun
( old use ) an offensive way of referring to sb who does not believe in what the speaker considers to be the true religion
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WORD ORIGIN
late 15th cent.: from French infidèle or Latin infidelis , from in- not + fidelis faithful (from fides faith, related to fidere to trust). The word originally denoted a person of a religion other than one's own, specifically a Muslim (to a Christian), a Christian (to a Muslim), or a Gentile (to a Jew).