1796, from French incroyable, literally "incredible" (15c.), from in- "not, opposite of, without" (see in- (1)) + croire "to believe," from Latin credere "to believe" (see credo). A name for the French fop or dandy of the period of the Directory (1795-1799). Said to be so called from their extravagant dress, and also, according to OED, from a favorite expression among them ("C'est vraiment incroyable").
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."