Online Etymology Dictionary

medio-: word-forming element from comb. form of Latin medius "middle" (see medial (adj.)).
mediocre (adj.): 1580s, from Middle French médiocre (16c.), from Latin mediocris "of middling height or state, moderate, ordinary," figuratively "mediocre, mean, inferior," originally "halfway up a mountain," from medius "middle" (see medial (adj.)) + ocris "jagged mountain" (cf. Greek okris "peak, point," Welsh ochr "corner, border," Latin acer "sharp;" see acrid). As a noun, "medicore thing or person," by 1834.
mediocritization (n.): 1917 (Will Durant), noun of state or action from mediocritize.
mediocritize (v.): 1854 (implied in mediocritizing); see mediocrity + -ize. Related: Mediocritized.
mediocrity (n.): early 15c., "moderation; intermediate state or amount," from Middle French médiocrité and directly from Latin mediocritatem (nominative mediocritas) "a middle state, middling condition, medium," from mediocris (see mediocre). Neutral at first; disparaging sense began to predominate from late 16c. The meaning "person of mediocre abilities or attainments" is from 1690s. Before the tinge of disparagement crept in, another name for the Golden Mean was golden mediocrity.
meditate (v.): 1580s, "to ponder," back-formation from meditation, or else from Latin meditatus, past participle of meditari (see meditation). Related: Meditated; meditating.
meditation (n.): c.1200, "contemplation; devout preoccupation; devotions, prayer," from Old French meditacion "thought, reflection, study," and directly from Latin meditationem (nominative meditatio) "a thinking over, meditation," noun of action from past participle stem of meditari "to meditate, think over, reflect, consider," frequentative form from PIE root *med- "to measure, limit, consider, advise, take appropriate measures" (cf. Greek medesthai "think about," medon "ruler;" Latin modus "measure, manner," modestus "moderate," modernus "modern," mederi "to heal," medicus "physician;" Sanskrit midiur "I judge, estimate;" Welsh meddwl "mind, thinking;" Gothic miton, Old English metan "to measure;" also see medical).

Meaning "discourse on a subject" is early 14c.; meaning "act of meditating, continuous calm thought upon some subject" is from late 14c. The Latin verb also had stronger senses: "plan, devise, practice, rehearse, study."
meditative (adj.): 1650s, from Late Latin meditativus, from meditat-, past participle stem of Latin meditari (see meditation). Related: Meditatively; meditativeness.
Mediterranean: "the sea between southern Europe and northern Africa," c.1400, from Late Latin Mediterraneum mare "Mediterranean Sea" (7c.), from Latin mediterraneus "midland;" the original sense being of "sea in the middle of the earth," from medius "middle" (see medial (adj.)) + terra "land, earth" (see terrain). The Old English name was Wendel-sæ, so called for the Vandals, Germanic tribe that settled on the southwest coast of it after the fall of Rome. The noun meaning "a person of Mediterranean race" is from 1888.
medium (n.): 1580s, "a middle ground, quality, or degree," from Latin medium "the middle, midst, center; interval," noun use of neuter of adjective medius (see medial (adj.)). Meaning "intermediate agency, channel of communication" is from c.1600. That of "person who conveys spiritual messages" first recorded 1853, from notion of "substance through which something is conveyed." Artistic sense (oil, watercolors, etc.) is from 1854. Happy medium is the "golden mean," Horace's aurea mediocritas.
medium (adj.): 1660s, "average," from medium (n.). The Latin adjective was medius. Meaning "intermediate" is from 1796. As a size designation from 1711. as a designation of cooked meat, it is attested from 1931, short for medium-rare (1881).
medlar (n.): "small fruit-bearing tree," mid-14c. (in reference to the fruit itself), from Old French medler, variant of mesple, from Latin mespila "fruit of the medlar," from Greek mespilion, a foreign word of unknown origin. The Old English name was openærs, literally "open-arse."
medley (n.): c.1300, "hand-to-hand combat," from Old French medlee, variant of meslee (see meddle). Meaning "combination, mixture" is from mid-15c.; that of "musical combination consisting of diverse parts" is from 1620s.
medulla (n.): hindmost segment of the brain, 1650s, from Latin medulla, literally "marrow," also "pith of plants," of unknown origin, perhaps related to or influenced by medius "middle" (but cf. also Old Irish smiur, Welsh mer "marrow"). The word was used in the Latin senses in Middle English. Related: Medular; medullary.
medusa (n.): "jellyfish," 1758, as genus name, from the name of one of the three Gorgons with snakes for hair, whose glance turned to stone him who looked upon it (attested in English from late 14c.). Her name is from Greek Medousa, literally "guardian," fem. present participle of the verb medein "to protect, rule over" (see Medea). The zoological name was chosen by Linnæus, suggested by the creature's long tentacles. Related: Medusoid.
meek (adj.): c.1200, "gentle, quiet, unaggressive; benevolent, kind; courteous, humble, unassuming;" of a woman, "modest," from a Scandinavian source (cf. Old Norse mjukr "soft, pliant, gentle"), from Proto-Germanic *meukaz (cf. Gothic muka-modei "humility," Dutch muik "soft"), of uncertain origin, perhaps from PIE *meug- "slippery, slimy." In the Bible, it translates Latin mansuetus from Vulgate (see mansuetude). Sense of "submissive" is from mid-14c.
meek (n.): "those who are meek," c.1200, from meek (adj.).
meekly (adv.): c.1200, from meek (adj.) + -ly (2).
meekness (n.): c.1200, meknesse; see meek (adj.) + -ness.
meerkat (n.): late 15c., "monkey," from Dutch meerkat "monkey" (related to Old High German mericazza), apparently from meer "lake" + kat "cat." But cf. Hindi markat, Sanskrit markata "ape," which might serve as a source of a Teutonic folk-etymology, even though the word was in Germanic before any known direct contact with India. First applied to the small South African mammals in 1801.
meerschaum (n.): type of soft white clay, 1784; from 1789 as "tobacco pipe with a bowl made of meerschaum clay," from German Meerschaum, literally "sea-foam," so called from its frothy appearance; from Old High German mari "sea" (see mere (n.)) + scum "scum" (see skim (v.)). A loan-translation of Latin spuma maris, itself said to be a loan translation of Greek halos akhne, from Persian kaf-i-darya.
meet (v.): Old English metan "to find, find out; fall in with, encounter; obtain," from Proto-Germanic *motjan (cf. Old Norse mæta, Old Frisian meta, Old Saxon motian "to meet," Gothic gamotijan), from PIE root *mod- "to meet, assemble." Related to Old English gemot "meeting." Meaning "to assemble" is from 1520s. Of things, "to come into contact," c.1300. Related: Met; meeting. To meet (someone) halfway in the figurative sense is from 1620s.
meet (adj.): "proper, fitting," Old English gemæte, Anglian *gemete, "suitable, having the same dimensions," from Proto-Germanic *ga-mætijaz (cf. Old Norse mætr, Old High German gimagi, German gemäß "suitable"), from collective prefix *ga- + PIE *med- "to measure" (see medical (adj.)). The basic formation is thus the same as that of commensurate.
meet (n.): 1831 in the sporting sense, originally of gatherings for hunting, from meet (v.).
meeting (n.): "action of coming together," Old English gemeting, verbal noun from meet (v.). Meaning "gathering of people for discussion, etc." is from 1510s. In 17c., it was applied generally to worship assemblies of nonconformists, but this now is retained mostly by Quakers.
meeting-house (n.): also meetinghouse, 1630s, from meeting (n.) + house (n.).
Meg: fem. proper name; before the late 20c. rise in popularity of Megan it typically was a pet form of Margaret, and was "used dial. to indicate a hoyden, coarse woman, etc." [OED]
mega-: before vowels meg-, word-forming element often meaning "large, great," but in precise scientific language "one million" (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE *meg- "great" (cf. Latin magnus, Old English micel; see mickle). Mega began to be used alone as an adjective by 1982.
High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]
megabucks (n.): 1946, originally "one million dollars," from mega- in the scientific sense + slang buck (n.) "dollar." A jocular coinage of U.S. scientists working on expensive atomic research.
megabyte (n.): 1972, from mega- + byte.
The Sussex team has run the Forrester/Meadows models more than 1000 times on the UK's most powerful computer (the giant two-megabyte IBM 370/165 at Harwell). ["New Scientist," May 4, 1972]
megacity (n.): also mega-city, 1968, from mega- + city.
megacycle (n.): 1928, from mega- + cycle (n.).
megadeath (n.): 1953, from mega- in scientific sense (one million) + death (n.). The death of one million persons, as a measure of the effectiveness of nuclear weapons. The resulting pile of dead bodies would be a megacorpse, according to writings on the topic.
megahertz (n.): 1941, from mega- + Hertz.
megalith (n.): huge prehistoric stone, 1853, back-formation from megalithic.
megalithic (adj.): 1836, from mega- "large" + lithos "stone" (see litho-) + -ic.
megalo-: word-forming element meaning "large, great, exaggerated," from comb. form of Greek megas "large, great" (stem megal-); see mickle.
megalocardia: 1893, from megalo- + cardia "heart" (see cardiac).
megalomania (n.): "delusions of greatness," 1866, from French mégalomanie; see megalo- + mania "madness."
megalomaniac: 1882 (n.), 1883 (adj.), from megalomania (q.v.).
The megalomaniac differs from the narcissist by the fact that he wishes to be powerful rather than charming, and seeks to be feared rather than loved. To this type belong many lunatics and most of the great men of history. [Bertrand Russell, "The Conquest of Happiness"]
megalomaniacal (adj.): 1884, from megalomaniac + -al (1).
megalopolis (n.): 1832, from comb. form of Greek megas (genitive megalou) "great" (see mickle) + polis "city" (see polis). The word was used in classical times as an epithet of great cities (Athens, Syracuse, Alexandria), and it also was the name of a former city in Arcadia.
megaphone (n.): 1878, coined (perhaps by Thomas Edison, who invented it) from Greek megas "great" (see mega-) + phone "voice" (see fame (n.)). Related: Megaphonic. In Greek, megalophonia meant "grandiloquence," megalophonos "loud-voiced."
megapixel (n.): by 1977, from mega- + pixel.
megaspore (n.): 1857, from mega- + spore.
megaton (n.): unit of explosive power equal to one million tons of TNT, 1952, from mega- + ton. Related: Megatonnage.
megavolt (n.): 1868, from mega- + volt.
megawatt (v.): 1900, from mega- + watt.
megillah (n.): "long, tedious, complicated story," 1957, from Yiddish (e.g. a gantse Megillah "a whole megillah"), literally "roll, scroll," collective name of the five Old Testament books appointed to be read on certain feast days, from Hebrew meghillah, from galal "he rolled, unfolded." The slang use is in reference to the length of the text.
megrim: see migraine.