Online Etymology Dictionary

Mozarab (n.): "assimilated Christian in Moorish Spain," one who was allowed to continue practicing his religion in exchange for political allegiance, from Spanish Mozarabe "would-be Arab," from Arabic mostarib, from a desiderative verbal form of Arab.
mozzarella (n.): 1911, Italian cheese originally made in Naples area, from Italian mozzarella, diminutive of mozza, a kind of cheese, from mozzare "to cut off," from mozzo "blunt," from Vulgar Latin *mutius "cut off, blunted."
mpg: originally m.p.g., abbreviation of miles per gallon, attested from 1912.
mph: also m.p.h., abbreviation of miles per hour, attested from 1887.
Mr.: mid-15c., abbreviation of master (n.); also see mister. Used from 1814 with a following noun or adjective, to denote "the exemplar or embodiment of that quality" (as in Mr. Right "the only man a woman wishes to marry," 1826; Mr. Fix-It, 1912; Mr. Big, 1940). The plural Messrs. (1779) is an abbreviation of French messieurs, plural of monsieur, used in English to supply the plural of Mr., which is lacking.
Mrs.: 1580s, abbreviation of mistress (q.v.), originally in all uses of that word. The plural Mmes. is an abbreviation of French mesdames, plural of madame, used in English to serve as the plural of Mrs., which is lacking. Pronunciation "missis" was considered vulgar at least into 18c. (cf missus). The Mrs. "one's wife" is from 1920.
Ms.: (plural Mses.), 1949, considered a blend of Miss and Mrs.
MS.: abbreviation of Latin manu scriptum (see manuscript); the plural is MSS, after the custom in Modern Latin.
Mstislav: Slavic masc. proper name, literally "vengeful fame," from Russian mstit' "to take revenge," from Proto-Slavic *misti "revenge," *mistiti "to take revenge," from PIE *mit-ti-, extended form of root *mei- (1) "to change, go, move" (see mutable); for second element, see Slav.
much (adj.): c. 1200, worn down by loss of unaccented last syllable from Middle English muchel "large, much," from Old English micel "great in amount or extent," from Proto-Germanic *mekilaz, from PIE *meg- "great" (see mickle). As a noun and an adverb, from c. 1200. For vowel evolution, see bury.
muchly (adv.): mid-12c., from much + -ly. Middle English used simply much as an adverb.
muchness (n.): late 14c., from much + -ness. Earlier was Middle English muchelnesse (c. 1200).
mucilage (n.): late 14c., mussillage, "viscous substance found in vegetable material," from Old French mucilage (14c.), from Late Latin mucilago "musty or moldy juice" (4c.), from Latin mucere "be musty or moldy," from mucus "mucus" (see mucus). Meaning "adhesive" is first attested 1859.
mucilaginous (adj.): early 15c., "viscous, sticky," from Medieval Latin muscilaginosus, from Late Latin mucilaginosus, from mucillago (see mucilage). Related: Mucilaginously.
muck (n.): mid-13c., "cow dung and vegetable matter spread as manure," from a Scandinavian source such as Old Norse myki, mykr "cow dung," Danish møg; from Proto-Germanic *muk-, *meuk- "soft." Meaning "unclean matter generally" is from c. 1300. Muck-sweat first attested 1690s.
muck (v.): late 14c., "to dig in the ground," also "to remove manure," early 15c., "to spread manure, cover with muck," from muck (n.). Meaning "to make dirty" is from 1832; in the figurative sense, "to make a mess of," it is from 1886; to muck about "mess around" is from 1856. Related: Mucked; mucking.
muck-a-muck (n.): "(self-)important person," 1912, from Chinook jargon, literally "to eat; food." Also mucky-muck; muckety-muck.
muckluck (n.): also mukluk, 1868, "sealskin, sealskin boots" from Eskimo maklak "large seal, sealskin." Meaning "canvas boots that resemble Eskimo ones" is from 1962.
muckraker (n.): c. 1600, "one who rakes muck," from muck (n.) + agent noun from rake (v.). Meaning "one who inquires into and publishes scandal and allegations of corruption among political and business leaders," popularized 1906 in speech by President Theodore Roosevelt, in reference to "man ... with a Muckrake in his hand" in Bunyan's "Pilgrim's Progress" (1684) who seeks worldly gain by raking filth.
The men with the muck-rakes are often indispensable to the well-being of society, but only if they know when to stop raking the muck. [T. Roosevelt, quoted in "Cincinnati Enquirer," April 15, 1906.]
Muckrake in sense "person who hunts scandal" is attested from 1872. To muckrake (v.) in the literal sense is from 1879; figuratively from 1910. Related: Muckraking.
mucose (adj.): 1731, from Latin mucosus (see mucous).
mucous (adj.): 1640s (replacing mucilaginous), from Latin mucosus "slimy, mucous," from mucus (see mucus). Related: mucosity.
mucus (n.): 1660s (replacing Middle English mucilage), from Latin mucus "slime, mold, mucus of the nose, snot," from PIE root *meug- "slippery, slimy," with derivatives referring to wet or slimy substances or conditions (source also of Latin emungere "to sneeze out, blow one's nose," mucere "be moldy or musty," Greek myssesthai "to blow the nose," myxa "mucus," mykes "fungus," Sanskrit muncati "he releases"). Old English had horh, which may be imitative.
mud (n.): mid-14c., cognate with and probably from Middle Low German mudde, Middle Dutch modde "thick mud," from Proto-Germanic *mud- from PIE *(s)meu-/*mu- [Buck], found in many words denoting "wet" or "dirty" (source also of Greek mydos "damp, moisture," Old Irish muad "cloud," Polish muł "slime," Sanskrit mutra- "urine," Avestan muthra- "excrement, filth"); related to German Schmutz "dirt," which also is used for "mud" in roads, etc., to avoid dreck, which originally meant "excrement." Welsh mwd is from English. Replaced native fen.

Meaning "lowest or worst of anything" is from 1580s. As a word for "coffee," it is hobo slang from 1925; as a word for "opium" from 1922. To throw or hurl mud "make disgraceful accusations" is from 1762. To say (one's) name is mud and mean "(one) is discredited" is first recorded 1823, from mud in obsolete sense of "a stupid twaddling fellow" (1708). Mud in your eye as a toast recorded from 1912, American English. Mud puppy "salamander" is from 1889, American English; mud bath is from 1798; mud pie is from 1788.
mud-flap (n.): 1903, from mud (n.) + flap (n.).
mudder (n.): "horse that runs well in muddy conditions," 1903, from mud (n.).
muddle (v.): 1590s, "destroy the clarity of" (a transferred sense); literal sense ("to bathe in mud") is from c. 1600; perhaps frequentative formation from mud, or from Dutch moddelen "to make (water) muddy," from the same Proto-Germanic source. Sense of "to make muddy" is from 1670s; that of "make confused" first recorded 1680s. Meaning "to bungle" is from 1885. Related: Muddled; muddling.
muddle (n.): 1818, from muddle (v.).
muddy (adj.): late 13c., from mud + -y (2). Big Muddy in reference to the Missouri or Mississippi rivers is first recorded 1825.
muddy (v.): "to make muddy," c. 1600, from muddy (adj.). Related: Muddied; muddying.
mudhole (n.): 1780, from mud (n.) + hole (n.).
mudsill (n.): 1680s, "lowest sill of a house," from mud + sill. The word entered U.S. political history in a speech by James M. Hammond of South Carolina, March 4, 1858, in U.S. Senate, alluding scornfully to the very mudsills of society, and the term subsequently was embraced by Northern workers in the pre-Civil War sectional rivalry.
muenster (n.): type of cheese, 1902, from Münster, mountain valley in Alsace, where it is made; the place name is German, literally "minster."
muesli (n.): breakfast dish of oats, fruit, milk, 1926, from Swiss-German, from Old High German muos "meal, mush-like food," from Proto-Germanic *mod-sa-, from PIE root *mad- "moist, wet," with derivatives referring to various qualities of food (see mast (n.2)).
muezzin (n.): "official who calls Muslims to prayer from the minaret of a mosque," 1580s, from Arabic muadhdhin, properly active participle of adhdhana, frequentative of adhanna "he proclaimed," from uthn "ear." Compare Hebrew he'ezin "he gave ear, heard," from ozen "ear." English spelling is from dialectal use of -z- for -dh-.
muff (n.): "warm covering for the hands," 1590s, from Dutch mof "a muff," shortened from Middle Dutch moffel "mitten, muff," from Middle French moufle "mitten," from Old French mofle "thick glove, large mitten, handcuffs" (9c.), from Medieval Latin muffula "a muff," of unknown origin. In 17c.-18c. also worn by men. Meaning "vulva and pubic hair" is from 1690s; muff-diver "one who performs cunnilingus" is from 1935.
muff (v.): "to bungle," 1827, pugilism slang, probably related to muff (n.) "awkward person" (1837), perhaps from muff (n.) on notion of someone clumsy because his hands are in a muff. Related: Muffed; muffing.
muffin (n.): "light, small cake made with eggs," 1703, moofin, possibly from Low German muffen, plural of muffe "small cake;" or somehow connected with Old French moflet "soft, tender" (said of bread). Muffin top in reference to waistline bulge over tight, low jeans is attested by 2005, from resemblance to baked muffins from a tin.
muffle (v.): early 15c., "to cover or wrap (something) to conceal or protect," perhaps from Middle French mofler "to stuff," from Old French moufle "thick glove, muff" (compare Old French enmoufle "wrapped up"); see muff (n.). Meaning "wrap something up to deaden sound" first recorded 1761. Related: Muffled; muffling.
muffle (n.): "thing that muffles," 1560s, from muffle (v.).
muffler (n.): 1530s as a kind of wrap for the throat, agent noun from muffle (v.); as an automobile exhaust system silencer, it is attested from 1895.
mufti (n.): 1580s, muphtie "official head of the state religion in Turkey," from Arabic mufti "judge," active participle of afta "to give," conjugated form of fata "he gave a (legal) decision" (compare fatwa). Sense of "ordinary clothes (not in uniform)" is from 1816, of unknown origin, perhaps from mufti's costume of robes and slippers in stage plays, which was felt to resemble plain clothes.
mug (n.1): "drinking vessel," 1560s, "bowl, pot, jug," of unknown origin, perhaps from Scandinavian (compare Swedish mugg "mug, jug," Norwegian mugge "pitcher, open can for warm drinks"), or Low German mokke, mukke "mug," also of unknown origin.
mug (n.2): "a person's face," 1708, possibly from mug (n.1), on notion of drinking mugs shaped like grotesque faces. Sense of "portrait or photograph in police records" (as in mug shot, 1950) had emerged by 1887. Hence, also, "a person" (especially "a criminal"), 1890.
mug (v.1): "to beat up," 1818, originally "to strike the face" (in pugilism), from mug (n.2). The general meaning "attack" is first attested 1846, and "attack to rob" is from 1864. Perhaps influenced by thieves' slang mug "dupe, fool, sucker" (1851). Related: Mugged; mugging.
mug (v.2): "make exaggerated facial expressions," 1855, originally theatrical slang, from mug (n.2). Related: Mugged; mugging.
mugger (n.): 1865, agent noun from mug (v.1).
mugging (n.): "violent physical attack," 1846, verbal noun from mug (v.1). As "grimmacing, making faces," 1937, from mug (v.2).
muggins (n.): "fool, simpleton," 1855, apparently from the surname, perhaps influenced by slang mug "dupe, fool" (1859; see mug (n.2)).
muggle (n.): "marijuana, a joint," 1926, apparently originally a New Orleans word, of unknown origin.
Muggletonian (n.): 1660s, member of the sect founded by English tailor Lodowicke Muggleton (1609-1698).