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.).