Online Etymology Dictionary

monomania (n.): "insanity in regard to a single subject or class of subjects," 1820, probably on model of earlier French monomanie, from Modern Latin monomania, from Greek monos "single, alone" (see mono-) + mania (see mania).
Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]
monomaniac (n.): 1833; see monomania + maniac. Related: Monomaniacal.
monomer (n.): 1914, from mono- + Greek meros "part" (see merit (n.)). Related: Monomerous.
monometallic (adj.): 1877 in currency sense, from mono- + metallic.
mononuclear (adj.): 1886; see mono- + nuclear.
mononucleosis (n.): 1920, coined from mononuclear + Modern Latin -osis "abnormal condition."
monophonic (adj.): of recordings, broadcasts, etc., "not stereo, having only one output signal," 1958, coined to be an opposite of stereophonic; from mono- + -phonic, from Greek phone "sound, voice," from PIE root *bha- (2) "to speak, tell, say" (see fame (n.)).
Monophysite: 1690s, from Church Latin Monophysita, from Greek monophysites, from monos "single, alone" (see mono-) + physis "nature" (see physics). Christian (regarded in the West as a heretic) who believes there is only one nature in the person of Jesus Christ. Now comprising Coptic, Armenian, Abyssinian and Jacobite churches.
monoplane (n.): 1907, a hybrid coined from mono- + second element of aeroplane. In old planes the wings formed a single surface running across the fuselage.
monopolistic (adj.): 1858; see monopoly + -istic.
monopolize (v.): 1610s; see monopoly + -ize. Figurative use from 1620s. Related: Monopolized; monopolizing; monopolization.
monopoly (n.): "exclusive control of a commodity or trade," 1530s, from Latin monopolium, from Greek monopolion "right of exclusive sale," from mono- + polein "to sell," from PIE root *pel- (5) "to sell" (source also of Sanskrit panate "barters, purchases," Lithuanian pelnas "gain," Old Church Slavonic splenu, Russian polon "prey, booty," Old Norse falr, Dutch veil, German feil "for sale, venal").

Alternative form monopole (1540s, from the Old French form of the word) was common in 16c. The popular board game, invented by Charles Darrow, is from 1935. Monopoly money "unreal currency" is attested from 1972, in reference to the paper used in the game.
monopolylogue (n.): "entertainment in which one actor performs as many characters," 1824, from mono- + poly- + -logue.
monorail (n.): 1897, a hybrid coined from mono- + rail (n.1).
monosyllabic (adj.): 1824, of languages; 1828, of words; 1870, of persons, from monosyllable + -ic. Earlier form was monosyllabical (1680s, of words). Related: Monosyllabically.
monosyllable (n.): 1530s, from Latin monosyllabus "of one syllable," from Greek monosyllabos, from monos "single, alone" (see mono-) + syllabe "syllable" (see syllable).
monotheism (n.): "belief that there is but one god," 1650s, from Greek mono-, comb. form of monos "single, alone" (see mono-) + theos "a god" (see theo-).
monotheist (n.): 1670s, from monotheism + -ist.
monotheistic (adj.): 1846, from monotheist + -ic.
monotone (n.): "unvarying tone in music or speaking," 1640s; see monotony. OED says use of the word as a noun "is peculiar to Eng." Related: Monotonic; monotonically.
monotonous (adj.): 1750, of sound, from Greek monotonos "of one tone" (see monotony). Transferred and figurative use, "lacking in variety, uninteresting," is from 1783. Related: Monotonously.
monotony (n.): 1706, originally in transferred sense of "wearisome, tiresome," from French monotonie (1670s), from Greek monotonia "sameness of tone, monotony," from monotonos "monotonous, of one tone," from monos "single, alone" (see mono-) + tonos "tone" (see tenet). Literal sense of "sameness of tone or pitch" in English is from 1724.
monotype (n.): 1881 in biology; 1882 in printers' arts; 1893 as a brand name of typesetting machine; see mono- + type.
monoxide (n.): "oxide with one oxygen atom in each molecule," 1869, from mono- + oxide.
Monroe Doctrine: 1848, in reference to principles of policy contained in the message of U.S. President James Monroe to Congress on Dec. 2, 1823.
mons (n.): from Latin mons (plural montes) "mountain" (see mount (n.1)); used in English in various anatomical senses, especially mons Veneris "mountains of Love," fleshy eminence atop the vaginal opening, 1690s; often mons for short.
monseigneur (n.): c. 1600, from French monseigneur (12c.), title of honor equivalent to "my lord," from mon "my" (from Latin meum) + seigneur "lord," from Latin seniorem, accusative of senior "older" (see senior (adj.)). Plural messeigneurs.
monsieur (n.): 1510s, from French monsieur, from mon sieur "my lord," from sieur "lord," shortened form of seigneur (see monseigneur) It was the historical title for the second son or next younger brother of the king of France.
monsignor (n.): title conferred on some prelates, 1640s, from Italian monsignore "my lord," formed on model of French monseigneur (see monseigneur) from equivalent elements in Italian.
monsoon (n.): 1580s, "trade wind of the Indian Ocean," from Dutch monssoen, from Portuguese monçao, from Arabic mawsim "time of year, appropriate season" (for a voyage, pilgrimage, etc.), from wasama "he marked." When it blows from the southwest (April through October) it brings heavy rain, hence "heavy episode of rainfall during the rainy season" (1747).
monster (n.): early 14c., "malformed animal or human, creature afflicted with a birth defect," from Old French monstre, mostre "monster, monstrosity" (12c.), and directly from Latin monstrum "divine omen, portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," from root of monere "warn" (see monitor (n.)). Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended by late 14c. to imaginary animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1520s; sense of "person of inhuman cruelty or wickedness" is from 1550s. As an adjective, "of extraordinary size," from 1837. In Old English, the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression."
monstrosity (n.): 1550s, "abnormality of growth," from Late Latin monstrositas "strangeness," from Latin monstrosus, a collateral form of monstruosus (source of French monstruosité); see monster. Earlier form was monstruosity (c. 1400). Sense of "quality of being monstrous" is first recorded 1650s. Meaning "a monster" is attested from 1640s.
monstrous (adj.): mid-15c., "unnatural, deviating from the natural order, hideous," from Middle French monstrueux, from Latin monstruosus "strange, unnatural, monstrous," from monstrum (see monster). Meaning "enormous" is from c. 1500; that of "outrageously wrong" is from 1570s. Earlier form monstruous (late 14c., from Old French monstruous) was "very common in the 16th c." [OED].
montage (n.): 1929, from French montage "a mounting," from Old French monter "to go up, mount" (see mount (v.)). Originally a term in cinematography.
montagnard (n.): "mountaineer, highlander," 1842, from French montagnard, from montagne (12c., see mountain).
Montana: U.S. state, from Latinized form of Spanish montaña "mountain," from Latin mont-, stem of mons (see mountain). Proposed 1864 by U.S. Rep. James H. Ashley of Ohio when it was created as a territory from Nebraska Territory, in reference to the Rocky Mountains, which however traverse only one end of it. Admitted as a state 1889. Related: Montanan.
Montanist (n.): mid-15c., millenarian and severely ascetic sect that believed in continual direct inspiration of the spirit and offered prominent church roles to women, from Montanus, Christian-inspired prophet in the wilds of Phrygia c. 160 C.E. The heresy persisted into the 6c. and helped bring prophecy into disrepute in the established Church. Related: Montanism.
monte (n.): gambling card game, 1824, from Spanish monte "mountain," from Latin montem (nominative mons), see mount (n.1). So called from the heap of cards left after dealing. A favorite in California during the gold rush years. The three-card form (first attested 1877) is of Mexican origin.
Monte Carlo: Italian, literally "Charles's Mountain," founded 1866 and named for Charles III of Monaco (1818-1889). The car rally there dates to 1911.
Monte Carlo fallacy: 1957, named for resort in Monaco famous for its gambling casinos. The fallacy of thinking that the probability of a particular outcome rises with the successive number of opposite outcomes. Contrary to the Monte Carlo fallacy, if the roulette wheel stops on black 99 times in a row, the chances that the 100th spin will be red are still just under 50-50.
Montenegro: Adriatic coastal nation, from Venetian Italian (Tuscan monte nero), literally "black mountain," a loan-translation of the local Slavonic name, Crnagora. Related: Montenegrine.
Monterey: city in California, U.S., formerly the Spanish Pacific capital, named for the bay, which was named 1603 for Spanish colonist and viceroy of New Spain Conde de Monterrey. The Monterrey in Mexico also is named for him.
Montessori: 1912, in reference to the system of education through free but guided play, devised 1907 by Italian educationist Maria Montessori (1870-1952).
Montezuma's revenge: "severe intestinal infection," such as often suffered by non-natives in Mexico, 1962, in reference to Montezuma II (1466-1520), Aztec ruler at the time of the Spanish conquest of Mexico.
month (n.): Old English monað, from Proto-Germanic *menoth- (source also of Old Saxon manoth, Old Frisian monath, Middle Dutch manet, Dutch maand, Old High German manod, German Monat, Old Norse manaðr, Gothic menoþs "month"), related to *menon- "moon" (see moon (n.); the month was calculated from lunar phases). Its cognates mean only "month" in the Romance languages, but in Germanic generally continue to do double duty. Phrase a month of Sundays "a very long time" is from 1832 (roughly 7 and a half months, but never used literally).
monthly (adv.): 1530s, from month + -ly (2). As an adjective from 1570s. Old English had monaþlic, but the modern words seem to be separate formations.
Montmartre: district in Paris, from Latin Mons Martyrum "Martyrs' Mount," in reference to St. Denis, first bishop of Paris, beheaded here with two companions in 258. The older name was Mons Mercurii.
Montreal: city in Canada, originally Ville Marie de Montréal, settled by the French 1642, named for the hill on which it was built, Mont Réal, in French literally "royal mount;" named 1534 by Jacques Cartier in honor of Francis I. Related: Montrealer.
monument (n.): late 13c., "a sepulchre," from Old French monument "grave, tomb, monument," and directly from Latin monumentum "a monument, memorial structure, statue; votive offering; tomb; memorial record," literally "something that reminds," from monere "to remind, warn" (see monitor (n.)). Sense of "structure or edifice to commemorate a notable person, action, or event" first attested c. 1600.
monumental (adj.): c. 1600, "pertaining to a monument," from Late Latin monumentalis "pertaining to a monument," from monumentum (see monument). From 1650s in the loose sense of "vast, stupendous." Related: Monumentally.