Etymology
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mercy (n.)

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;" in Medieval Latin "thanks; grace"), from merx (genitive mercis) "wares, merchandise" (see market (n.)). In Church Latin (6c.) it was given a specific application to the heavenly reward earned by those who show kindness to the helpless and those from whom no requital can be expected.

Meaning "disposition to forgive or show compassion" is attested from early 13c. Sense of "an act or exercise of forbearance or good will" is from c. 1300. As an interjection, attested from mid-13c. (short for may God have mercy, have mercy on me, etc.).  Many of the English senses are found earlier in French, but in French the word largely has been superseded by miséricorde except as a word of thanks. Sense of "discretionary action" (as in at (one's) mercy) is from mid-14c. Seat of mercy "golden covering of the Ark of the Covenant" (1530), hence "the throne of God," is Tyndale's loan-translation of Luther's gnadenstuhl, an inexact translation of Latin propitiatorium, ultimately a rendering of Hebrew kapporeth, literally "propitiatory."

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merciless (adj.)

late 14c., "unfeeling, pitiless, cruel," from mercy + -less. Sense of "relentless" is from early 15c.; of inanimate things, from 1580s. Related: Mercilessly; mercilessness.

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merciful (adj.)

"exercising forbearance or pity; characterized by mercy, giving relief from danger, need, or suffering," mid-14c., from mercy + -ful. The earlier word was merciable (c. 1200). Related: Mercifully; mercifulness.

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Mercedes 

fem. proper name, from Spanish, abbreviation of Maria de las Mercedes "Mary of the Mercies," from plural of merced "mercy, grace," from Latin mercedem (nominative merces) "hire, pay, wage, salary; rent, income; a price for anything;" see mercy. The early Christians gave a spiritual meaning to the purely financial classical senses of the Latin word, which also, in its original senses, entered Middle English as mercede "wages" (late 14c.).

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amerce (v.)

"punishment by arbitrary or discretionary fine," 1215, earlier amercy, Anglo-French amercier "to fine," from merci "mercy, grace" (see mercy). The legal phrase estre a merci "to be at the mercy of" (a tribunal, etc.) was corrupted to estre amercié in an example of how an adverbial phrase in legalese can become a verb (compare abandon).

Frans hom ne seit amerciez pour petit forfet. [Magna Charta]

Related: Amercement; amerciable/amerceable.

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gramercy (interj.)
c. 1300, exclamation of thanks, later of surprise, from Old French grant-merci, gran merci "great thanks, many thanks," from gran (see grand (adj.)) + merci "reward, favor, thanks" (see mercy (n.)). Modern French merci "thank you" is a shortening of this.

New York City's Gramercy Park is named for the Gramercy Farm which once stood there; the first part of the name is an 18c. folk-etymology from Crommeshie Fly, the name of a former marsh or shallow pond that stood nearby, itself a mangling of New Netherlands Dutch Crommessie Vly, the first part of which represents either *Krom Moerasje "little crooked swamp" or *Krom Messje "little crooked knife," said to have been the name of a brook flowing into (or out of) the pond.
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Kyrie eleison 
early 13c., a Greek liturgical formula adopted untranslated into the Latin mass, literally "lord have mercy" (Psalms cxxii.3, Matthew xv.22, xvii.15, etc.). From kyrie, vocative of kyrios "lord, master" (see church (n.)) + eleeson, aorist imperative of eleo "I have pity on, show mercy to," from eleos "pity, mercy" (see alms). Hence, the corresponding part of a musical setting of the Mass or Anglican Communion.
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Miserere (n.)

c. 1200, "recitation of the 51st Psalm" (in Vulgate, the 50th), one of the "Penitential Psalms," so called from the phrase Miserere mei Deus "Have mercy upon me, O God," the opening line of it in the Vulgate, from Latin miserere "feel pity, have compassion, commiserate," second person singular imperative of misereri "to have mercy," from miser "wretched, pitiable" (see miser).

From 15c.-17c. it was used as an informal measure of time, "the time it takes to recite the Miserere." The musical settings of the psalm are noted for their striking effectiveness. The Latin verb also is in miserere mei "kind of severe colic ('iliac passion') accompanied by excruciating cramps and vomiting of excrement" (1610s); literally "have mercy on me."

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adrift (adv.)
"floating at random, at the mercy of currents," 1620s, from a- (1) "on" + drift (n.). Figurative use by 1680s.
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unsparing (adj.)
"showing no mercy," 1580s, from un- (1) "not" + sparing, attested from late 14c. as a present-participle adjective from spare (v.). Meaning "profuse" is from 1660s. Related: Unsparingly.
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