"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."