incroyable n.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

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.

Proto-Indo-European root meaning "heart."

It forms all or part of: accord; cardiac; cardio-; concord; core; cordial; courage; credence; credible; credit; credo; credulous; creed; discord; grant; heart; incroyable; megalocardia; miscreant; myocardium; pericarditis; pericardium; quarry (n.1) "what is hunted;" record; recreant; tachycardia.

It is the hypothetical source of/evidence for its existence is provided by: Greek kardia, Latin cor, Armenian sirt, Old Irish cride, Welsh craidd, Hittite kir, Lithuanian širdis, Russian serdce, Old English heorte, German Herz, Gothic hairto, "heart;" Breton kreiz "middle;" Old Church Slavonic sreda "middle."