The derivation
καρδία is the rare soul-word whose etymology is not in doubt — it is, on the contrary, one of the most securely reconstructed words in the Greek lexicon, an inheritance of immense antiquity. Where ψυχή, θυμός, νόος, and φρήν leave their three dictionaries quarrelling, here all three simply lay out the same vast cognate family with quiet agreement.
The base is a monosyllabic neuter, κῆρ (Homer's poetic "heart"), from Indo-European *ḱērd / *ḱr̥d- — and the cognates are a roll-call of the Indo-European world: Latin cor, cordis; Hittite kēr, gen. kardiaš; Lithuanian širdìs; Armenian sirt; Old Irish cride; Gothic hairtō; Old Church Slavonic srьdьce; Sanskrit hṛd- (Beekes, EDG s.v. καρδία; Frisk, GEW s.v. καρδία, both giving the full row). Beekes notes one famous wrinkle — the Indo-Iranian forms show a secondary h- where the law predicts ś-, "probably by contamination with another word," with the regular outcome preserved in Sanskrit śraddhā- "to trust" (literally "to put one's heart"). καρδία itself is the i-stem extension, formed like other body-part nouns in -ία (κοιλία "belly," ἀρτηρία "windpipe").
Where the dictionaries do correct the record is on the poets: the Homeric κέαρ, long taken as an archaic form, is, Chantraine and Frisk agree, "un faux archaïsme" — a fake antique minted by poets on the model of ἔαρ / ἦρι "spring."
The deepest note belongs to that Sanskrit cognate śraddhā-: Benveniste's analysis (retrieved here from the Oracle's Indo-European Language and Society) reads it as śrad + dhā-, "to place the heart" — faith itself as a setting-down of the heart before a god. The word for the organ and the word for trust are, at the Indo-European root, the same gesture.
Root
- *ḱērd- / *ḱr̥d- "heart" — secure, with the widest cognate distribution of any soul-word in this set (Beekes, Frisk, Chantraine all concur). Greek κῆρ / κέαρ the inherited neuter; καρδία the i-stem derivative.
In the corpus
106 occurrences across 15 works — Homeric at the core (Iliad 33, Odyssey 26, almost always in the metrically convenient epic form κραδίη; earliest κραδίην at Il. 1.225, Achilles to Agamemnon), and then carried forward by the prose anatomists: Aristotle's De Somno and De Sensu, where καρδία becomes the literal pumping organ and even the seat of perception. Set beside σῶμα's explosion in Plotinus, καρδία's restraint marks it as the word that stayed bodily while the soul-vocabulary went metaphysical.
The word's world
In the cluster of Homeric heart-words — κραδίη, ἦτορ, κῆρ — καρδία is the one most tied to the beating organ and to the rush of courage or fear through it. Snell collects the Iliadic formulae where the κραδίη fills with daring (Il. 21.547, "into his heart he cast boldness") and stands beside θυμός as the engine of valor (Il. 10.244; 16.266) — the heart and the spirit yoked in the same line (The Discovery of the Mind). Sullivan treats κραδίη, ἦτορ, and κῆρ together as a sub-family of the Homeric inner organs, each a site where feeling is located, not merely felt (Psychological and Ethical Ideas). Where ψυχή is the breath that survives and θυμός the heat that animates, καρδία is the drum in the chest that quickens — the most physical, and the most universally Indo-European, of the words the Greeks used for the soul.
Authorities: Beekes, Etymological Dictionary of Greek (Brill 2010) s.v. καρδία (scan p. 691, #2933); Chantraine, Dictionnaire étymologique de la langue grecque s.v. καρδία (scan pp. 511–512, #3660 + #3662, the κῆρ/κέαρ discussion); Frisk, Griechisches etymologisches Wörterbuch s.v. καρδία (scan pp. 819–820, #2797; Nachträge p. 2245, #7318). Interpretive sources retrieved from the Oracle library (Classical Philology & Ancient Thought): Snell, The Discovery of the Mind (1953); Sullivan, Psychological and Ethical Ideas (1995); Benveniste, Indo-European Language and Society (1973), on śraddhā. Cognate row also in Pokorny 579; Ernout–Meillet s.v. cor. Corpus figures: Logoi corpus, live. Receipt: soul-word-journey-v0.