英语词源字典 - 在线词源网站

faithless (adj.)[faithless 词源字典]: c. 1300, "unbelieving," from faith + -less. Meaning "insincere, deceptive" is mid-14c. Related: Faithlessly; faithlessness.[faithless etymology, faithless origin, 英语词源]
incredible (adj.): early 15c., "unbelievable," from Latin incredibilis "not to be believed," from in- "not" (see in- (1)) + credibilis "worthy of belief" (see credible). Used c. 1400 in a now-extinct sense of "unbelieving, incredulous." Related: Incredibly.
incredulous (adj.): "unbelieving," 1570s, from Latin incredulus "unbelieving, incredulous," from in- "not" (see in- (1)) + credulus (see credulous). Formerly also of religious beliefs. Related: Incredulously; incredulousness.
infidel: mid-15c. (adjective and noun), from Middle French infidèle, from Latin infidelis "unfaithful, not to be trusted," later "unbelieving," from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (see fidelity). In 15c. "a non-Christian" (especially a Saracen); later "one who does not believe in religion" (1520s). Also used to translate Arabic qafir, which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s; see kaffir).
miscreant (adj.): c. 1300, "non-Christian, pagan, infidel;" early 15c., "heretical, unbelieving," from Old French mescreant "disbelieving" (Modern French mécréant), from mes- "wrongly" (see mis- (2)) + creant, present participle of creire "believe," from Latin credere (see credo). Meaning "villainous" is from 1590s.
unfaithful (adj.): mid-14c., "acting falsely," from un- (1) "not" + faithful. In Middle English it also had a sense of "infidel, unbelieving, irreligious" (late 14c.). Sense of "not faithful in marriage" is attested from 1828. Related: Unfaithfully; unfaithfulness.