Online Etymology Dictionary

merciless (adj.): late 14c., see mercy + -less. Related: Mercilessly.
mercurial (adj.): late 14c., "pertaining to the planet Mercury" (see Mercury). Meaning "sprightly, volatile, quick" (1590s) is from supposed qualities of those born under the planet Mercury (they also are the qualities of the god Mercury), probably also partly by association with the qualities of quicksilver. A variant in this sense was mercurious (1590s). Related: Mercurially; mercuriality.
mercuric (adj.): 1828, in chemistry, from mercury + -ic.
Mercury: "the Roman god Mercury," mid-12c., from Latin Mercurius "Mercury," originally a god of tradesmen and thieves, from merx "merchandise" (see market (n.)); or perhaps [Klein, Tucker] from Etruscan and influenced by merx. Later he was associated with Greek Hermes. The planet closest to the sun so called in classical Latin (late 14c. in English). A hypothetical inhabitant of the planet was a Mercurean (1855) or a Mercurian (1868). For the metallic element, see mercury.
mercury (n.): silver-white fluid metallic element, late 14c., from Medieval Latin mercurius, from Latin Mercurius (see Mercury). Prepared from cinnabar, it was one of the seven metals (bodies terrestrial) known to the ancients, which were coupled in astrology and alchemy with the seven known heavenly bodies. This one probably so associated for its mobility. The others were Sun/gold, Moon/silver, Mars/iron, Saturn/lead, Jupiter/tin, Venus/copper. The Greek name for it was hydrargyros "liquid silver," which gives the element its symbol, Hg. Compare quicksilver.
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"), 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 In French largely superseded by miséricorde except as a word of thanks.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 gnadenstuhl, an inexact rendering of Hebrew kapporeth, literally "propitiatory."
merde (n.): also merd, "dung," late 15c., from French merde "feces, excrement, dirt" (13c.), from Latin merda "dung, ordure, excrement," of unknown origin. Naturalized in English through 17c., but subsequently lost and since mid-19c. (and especially since World War I) generally treated as a French word when used in English.
mere (adj.): c. 1400, "unmixed, pure," from Old French mier "pure" (of gold), "entire, total, complete," and directly from Latin merus "unmixed" (of wine), "pure; bare, naked;" figuratively "true, real, genuine," probably originally "clear, bright," from PIE *mer- "to gleam, glimmer, sparkle" (source also of Old English amerian "to purify," Old Irish emer "not clear," Sanskrit maricih "ray, beam," Greek marmarein "to gleam, glimmer"). Original sense of "nothing less than, absolute" (mid-15c., now only in vestiges such as mere folly) existed for centuries alongside opposite sense of "nothing more than" (1580s, as in a mere dream).
mere (n.): Old English mere "sea, ocean; lake, pool, pond, cistern," from Proto-Germanic *mari (source also of Old Norse marr, Old Saxon meri "sea," Middle Dutch maer, Dutch meer "lake, sea, pool," Old High German mari, German Meer "sea," Gothic marei "sea," mari-saiws "lake"), from PIE *mori- "sea" (source also of Latin mare, Old Church Slavonic morje, Russian more, Lithuanian mares, Old Irish muir, Welsh mor "sea," Gaulish Are-morici "people living near the sea").
merely (adv.): mid-15c., "entirely, purely," from mere (adj.) + -ly (2). Meaning "and nothing more" is from 1580s.
merengue (n.): popular dance, 1936, from Dominican Creole méringue, from French méringue (see meringue).
The Spanish word for this style of dance and music, merengue, literally means "meringue (the sweet dessert)" -- although it is unclear exactly how the dance might have come to be called "The Meringue." ["Spanish Word Histories and Mysteries," American Heritage Dictionaries, 2007]
merestone (n.): "stone serving as a landmark," Old English mærstan "boundary stone," from mære "boundary, object indicating a boundary," from Proto-Germanic *mairjo- (source also of Middle Dutch mere "boundary mark, stake," Old Norse -mæri "boundary, border-land"), related to Latin murus "wall" (see mural). Hence also meresman "man appointed to find boundaries" (of a parish, etc.).
meretricious (adj.): 1620s, "pertaining to harlots," from Latin meretricius "of or pertaining to prostitutes," from meretrix (genitive meretricis) "prostitute," literally "woman who earns money," from merere, mereri "to earn, gain" (see merit (n.)). Meaning "gaudily alluring" is from 1630s. Related: Meretriciously; meretriciousness.
merganser (n.): type of duck, 1752, coined in Modern Latin (1550s), from Latin mergus "waterfowl, diver" (from mergere "to dip, immerse;" see merge (v.)) + anser "goose" (see goose (n.)).
merge (v.): 1630s, "to plunge or sink in," from Latin mergere "to dip, dip in, immerse, plunge," probably rhotacized from *mezgo, from PIE *mezg- "to dip, plunge" (source also of Sanskrit majjati "dives under," Lithuanian mazgoju "to wash"). Legal sense of "absorb an estate, contract, etc. into another" is from 1726. Related: Merged; merging. As a noun, from 1805.
merger (n.): 1728 in legal sense, "extinguishment by absorption," from merge (v.), on analogy of French infinitives used as nouns (see waiver). From 1889 in the business sense; not common until c. 1926. General meaning "any act of merging" is from 1881.
meridian (n.): mid-14c., "noon," from Old French meridien "of the noon time, midday; the Meridian; southerner" (12c.), and directly from Latin meridianus "of midday, of noon, southerly, to the south," from meridies "noon, south," from meridie "at noon," altered by dissimilation from pre-Latin *medi die, locative of medius "mid-" (see medial (adj.)) + dies "day" (see diurnal). Cartographic sense first recorded late 14c. Figurative uses tend to suggest "point of highest development or fullest power."

The city in Mississippi, U.S., was settled 1854 (as Sowashee Station) at a railway junction and given its current name in 1860, supposedly by people who thought meridian meant "junction" (they perhaps confused the word with median).
meringue (n.): whites of eggs mixed with sugar, 1706, from French méringue (18c.), of unknown origin.
merino (n.): fine-wool breed of sheep, 1781, from Spanish merino, possibly from Arabic Merini, a Berber family or tribe of sheep farmers in northwest Africa whose animals were imported into Spain 14c.-15c. to improve local breeds. Or from or influenced by Latin majorinus, from major "greater," either in reference to size of the animals or from Spanish derivative merino (n.) "overseer of cattle pastures," also a title of judicial officers. Applied from early 19c. to the wool itself and to various articles made from it.
merism (n.): 1894 in biological sense "repetition of parts in living things;" earlier in rhetoric, "synecdoche in which totality is expressed by contrasting parts" (such as high and low, young and old); from Modern Latin merismus, from Greek merismos "a dividing, division, a partition," from merizein "to divide," from meros "part, share" (see merit (n.)). Related: Merismatic.
meristem (n.): "growing cellular tissues of plants," 1862, formed irregularly from Greek meristos "divided, divisible" (from merizein "to divide, distribute," from meros "a part, a share;" see merit (n.)) + ending from xylem, etc.
merit (n.): c. 1200, "spiritual credit" (for good works, etc.); c. 1300, "spiritual reward," from Old French merite "wages, pay, reward; thanks; merit, moral worth, that which assures divine pity," and directly from Latin meritum "a merit, service, kindness, benefit, favor; worth, value, importance," neuter of meritus, past participle of merere, meriri "to earn, deserve, acquire, gain," from PIE root *(s)mer- (2) "to allot, assign" (source also of Greek meros "part, lot," moira "share, fate," moros "fate, destiny, doom," Hittite mark "to divide" a sacrifice).

Sense of "worthiness, excellence" is from early 14c.; from late 14c. as "condition or conduct that deserves either reward or punishment;" also "a reward, benefit." Related: Merits. Merit system attested from 1880. Merit-monger was in common use 16c.-17c. in a sense roughly of "do-gooder."
merit (v.): late 15c., "to be entitled to," from Middle French meriter (Modern French mériter), from merite (n.), or directly from Latin meritare "to earn, yield," frequentative of mereri "to earn (money);" also "to serve as a soldier" (see merit (n.)). Related: Merited; meriting.
merited (adj.): "well-earned," c. 1600, past participle adjective from merit (v.).
meritless (adj.): 1590s, from merit (n.) + -less. Related: Meritlessly; meritlessness.
meritocracy (n.): coined 1958 by British sociologist Michael Young (1915-2002) and used in title of his book, "The Rise of the Meritocracy"; from merit (n.) + -cracy. Related: Meritocratic.
[Young's book] imagined an elite that got its position not from ancestry, but from test scores and effort. For him, meritocracy was a negative term; his spoof was a warning about the negative consequences of assigning social status based on formal educational qualifications, and showed how excluding from leadership anyone who couldn't jump through the educational hoops would create a new form of discrimination. And that's exactly what has happened. [Lani Guinier, interview, "New York Times," Feb. 7, 2015]
meritorious (adj.): early 15c., "deserving of divine grace," from Latin meritorius "that for which money is paid, that by which money is earned," from meritus, past participle of merere "to earn" (see merit (n.)). Related: Meritoriously; meritoriousness.
merkin (n.): "female pudenda," 1530s, apparently a variant of malkin in its sense of "mop." Meaning "artificial vagina or 'counterfeit hair for a woman's privy parts' " is attested from 1610s. According to "The Oxford Companion to the Body," the custom of wearing merkins dates from mid-15c., was associated with prostitutes, and was to disguise a want of pubic hair, shaved off either to exterminate body lice or evidence of venereal disease.
This put a strange Whim in his Head; which was, to get the hairy circle of [a prostitute's] Merkin .... This he dry'd well, and comb'd out, and then return'd to the Cardinall, telling him, he had brought St. Peter's Beard. [Alexander Smith, "A Complete History of the Lives and Robberies of the most notorious Highwaymen," 1714]
merle (n.): "blackbird," late 15c., from Old French merle "blackbird" (12c.), from Latin merulus "blackbird," from PIE *ams- "black, blackbird" (source also of Old English osle "blackbird;" see ouzel). The word owes its survival in modern times to its use by Scottish poets. The Latin word shows effects of rhotacism. It also is the source of Provençal and Spanish merla, Portuguese merlo, and Italian merla. Borrowed from French are Middle Dutch and German merle, Dutch meerle.
merlin (n.): small, strong European falcon, early 14c., from Anglo-French merilun, a shortened form of Old French esmerillon "merlin, small hawk" (12c., Modern French émerillon), from Frankish *smiril or some other Germanic source (compare Old High German smerlo, German Schmerl "merlin"). Spanish esmerejon, Italian smeriglio also are Germanic loan-words.
Merlin: sorcerer and soothsayer in Arthurian legends, from Old French form of Welsh Myrddhin, probably from Old Celtic *Mori-dunon, literally "of the sea-hill," from *mori "sea" (see mere (n.)) + dunom "hill" (see dune).
merlon (n.): "solid part of a battlement," 1704, from French merlon (17c.), from Italian merlone, augmentative of merlo "battlement," perhaps a contraction of mergola, diminutive of Latin mergae "two-pronged pitchfork."
mermaid (n.): mid-14c., mermayde, literally "maid of the sea," from Middle English mere "sea, lake" (see mere (n.)) + maid. Old English had equivalent merewif "water-witch" (see wife), meremenn "mermaid, siren." Tail-less in northern Europe; the fishy form is a medieval influence from classical sirens. A favorite sign of taverns and inns since at least early 15c. (in reference to the inn on Bread Street, Cheapside, London). Mermaid pie (1660s) was "a sucking pig baked whole in a crust."
merman (n.): c. 1600, literally "man of the sea," from first element in mermaid (q.v.) + man (n.).
mero-: before vowels mer-, word-forming element meaning "part, partial, fraction," from comb. form of Greek meros "part, fraction" (see merit (n.)).
Merovingian (adj.): 1690s, from French Mérovingien, from Medieval Latin Merovingi, "descendants of Meroveus," (mythical?) ancestor of the line of Frankish kings in Gaul (c.500-752) beginning with Clovis; Merovingi is a Latinization of his Germanic name (compare Old High German Mar-wig "famed-fight") with the Germanic patronymic suffix -ing.
merrily (adv.): Old English myriglice "pleasantly, melodiously;" see merry + -ly (2).
merriment (n.): 1570s, "comedic entertainment," from merry + -ment. General sense of "mirth" is from 1580s.
merry (adj.): Old English myrge "pleasing, agreeable, pleasant, sweet; pleasantly, melodiously," from Proto-Germanic *murgijaz, which probably originally meant "short-lasting," (compare Old High German murg "short," Gothic gamaurgjan "to shorten"), from PIE *mreghu- "short" (see brief (adj.)). The only exact cognate for meaning outside English was Middle Dutch mergelijc "joyful."

Connection to "pleasure" is likely via notion of "making time fly, that which makes the time seem to pass quickly" (compare German Kurzweil "pastime," literally "a short time;" Old Norse skemta "to amuse, entertain, amuse oneself," from skamt, neuter of skammr "short"). There also was a verbal form in Old English, myrgan "be merry, rejoice." For vowel evolution, see bury (v.).
Bot vchon enle we wolde were fyf, þe mo þe myryer. [c. 1300]
The word had much wider senses in Middle English, such as "pleasant-sounding" (of animal voices), "fine" (of weather), "handsome" (of dress), "pleasant-tasting" (of herbs). Merry-bout "an incident of sexual intercourse" was low slang from 1780. Merry-begot "illegitimate" (adj.), "bastard" (n.) is from 1785. Merrie England (now frequently satirical or ironic) is 14c. meri ingland, originally in a broader sense of "bountiful, prosperous." Merry Monday was a 16c. term for "the Monday before Shrove Tuesday" (Mardi Gras).
merry man (n.): "companion in arms, follower of a knight, outlaw, etc.," is attested from late 14c., from merry (adj.) + man (n.). Related: Merry men.
Merry Widow: 1907, from the English title of Franz Lehar's operetta "Die Lustige Witwe" (1905). "The Lusty Widow" would have been more etymological (see lust (n.)), but would have given the wrong impression in English. Meaning "a type of wide-brimmed hat" (popularized in the play) is attested from 1908.
merry-andrew (n.): "a buffoon; a zany; a jack-pudding" [Johnson], originally "mountebank's assistant," 1670s, from merry + masc. proper name Andrew, but there is no certain identification with an individual.
merry-go-round (n.): 1729, from merry (adj.) + go-round. Figurative use by 1838. Merry-totter (mid-15c.) was a Middle English name for a swing or see-saw. Also compare merry-go-down "strong ale" (c. 1500); merry-go-sorry "a mix of joy and sorrow" (1590s).
merrymaking (n.): also merry-making, 1714; see merry + make (v.). Related: Merry-maker (1827).
merrythought (n.): "wishbone," c. 1600, from merry (adj.) + thought. Also see wishbone.
mesa (n.): "high table land," 1759, from Spanish mesa "plateau," literally "table," from Latin mensa "table" (source of Rumanian masa, Old French moise "table").
mesalliance (n.): "marriage with a person of lower social position," 1782, from French mésalliance, from pejorative prefix mes- (from Latin mis-; see mis-) + alliance (see alliance).
mescal (n.): "plant of the genus Agave," found in deserts of Mexico and southwestern U.S., especially the American aloe, or maguey plant, 1702, from Mexican Spanish, from Nahuatl (Aztecan) mexcalli "fermented drink made from agave," from metl "agave" + ixcalli "stew." Meaning "intoxicating liquor from fermented juice of the agave" is attested in English from 1828. Also the name of a small desert cactus (peyote) found in northern Mexico and southern Texas (1885).
mescaline (n.): crystalline alkaloid, 1896, from German mezcalin (1896), so called because it originally was found in the buttons that grow atop the mescal cacti (see mescal). With chemical suffix -ine (2).
mesdames: plural of French madame (see madam).