late 12c., "God's forgiveness of his creatures' offenses," from Old French mercit, merci (9c.) "reward, gift; kindness, grace, pity," from Latin mercedem (nominative merces) "reward, wages, pay hire" (in Vulgar Latin "favor, pity"), from merx (genitive mercis) "wares, merchandise" (see market (n.)). In Church Latin (6c.) applied to the heavenly reward of those who show kindness to the helpless.
Meaning "disposition to forgive or show compassion" is attested from early 13c. As an interjection, attested from mid-13c. In French largely superseded by miséricorde except as a word of thanks. Seat of mercy "golden covering of the Ark of the Covenant" (1530) is Tyndale's loan-translation of Luther's