credible (adj.)

early 13c., "the Creed in the Church service," from Latin credo "I believe," the first word of the Apostles' and Nicene creeds, first person singular present indicative of credere "to believe," from PIE compound *kerd-dhe- "to believe," literally "to put one's heart" (source also of Old Irish cretim, Irish creidim, Welsh credu "I believe," Sanskrit śrad-dhā- "faith, confidence, devotion"), from PIE root *kerd- "heart." The nativized form is creed. General sense of "formula or statement of belief" is from 1580s.

"quality of being credible, capacity or condition of being believed, just claim to credit," 1590s, from Medieval Latin credibilitas, from Latin credibilis (see credible). Credibility gap is 1966, American English, in reference to official statements about the Vietnam War.