Cardiac
late Middle English (as a noun denoting heart disease): from French cardiaque or Latin cardiacus, from Greek kardiakos, from kardia ‘heart or upper opening of the stomach’. The adjective dates from the early 17th century.
wiktionary
From Middle French cardiaque, from Latin cardiacus, from Ancient Greek καρδιακός(kardiakós, “relating to the heart”), from καρδία(kardía, “heart”).
etymonline
cardiac (adj.)
"of or pertaining to the heart," c. 1600, from French cardiaque (14c.) or directly from Latin cardiacus, from Greek kardiakos "pertaining to the heart," from kardia "heart" (from PIE root *kerd- "heart"). Cardiac arrest is attested from 1950.
Greek kardia also could mean "stomach" and Latin cardiacus "pertaining to the stomach." This terminology continues somewhat in modern medicine. Confusion of heart and nearby digestive organs also is reflected in Breton kalon "heart," from Old French cauldun "bowels," and English heartburn for "indigestion."
