Etymology
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indigestion (n.)
late 14c., "difficulty or inability in digesting food," from Old French indigestion (13c.), from Late Latin indigestionem (nominative indigestio), from in- "not, opposite of" (see in- (1)) + digestionem "arrangement, distribution" (see digestion). An Old English word for it was unmeltung.
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pepsin (n.)

also pepsine, "fermin found in gastric juice, used medicinally for cases of indigestion," 1844, coined in German (Theodor Schwann, 1835) from Greek pepsis "digestion; a cooking," from stem pep- (see peptic) + -in (2).

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crudity (n.)

early 15c., of food, "quality of producing unnatural humors," from Old French crudité (14c.) and directly from Latin cruditatem (nominative cruditas) "indigestion," from crudus "rough; not cooked, raw, bloody" (see crude). From 1620s as "that which is crude;" 1630s as "quality or state of being crude."

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heartburn (n.)
mid-13c., herte-brine "lust," later "burning sensation in the esophagus, indigestion" (mid-15c.); see heart (n.) + burn (n.). Compare cardiac for confusion of "heart" and "stomach." A Middle English alternative was herte-brenning "anger, bitterness" (c. 1400), also "heartburn" (mid-15c.).
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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."
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