Etymology
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mare (n.3)

"night-goblin, incubus, oppressed sleep," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" (source also of Middle Low German mar, Middle Dutch mare, Old High German mara, German Mahr "incubus," Old Norse mara "nightmare, incubus"). This is from PIE *mora- "incubus" (source also of first element in Old Irish Morrigain "demoness of the corpses," literally "queen of the nightmare," also Bulgarian, Serbian mora, Czech mura, Polish zmora "incubus"). Also compare French cauchemar "nightmare," with first element from Old French caucher "to trample" and second element from Germanic.

All this is probably from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The word in English now survives only in nightmare (q.v.).

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mare (n.1)

"female of the horse or any other equine animal," Old English meare, also mere (Mercian), myre (West Saxon), fem. of mearh "horse," from Proto-Germanic *marhijo- "female horse" (source also of Old Saxon meriha, Old Norse merr, Old Frisian merrie, Dutch merrie, Old High German meriha, German Mähre "mare"), said to be of Gaulish origin (compare Irish and Gaelic marc, Welsh march, Breton marh "horse").

The fem. form is not recorded in Gothic, and there are no known cognates beyond Germanic and Celtic, so perhaps it is a word from a substrate language. The masc. forms have disappeared in English and German except as disguised in marshal (n.). In 14c. also "a bad woman, a slut," and, apparently, also "a rabbit." As the name of a throw in wrestling, it is attested from c. 1600. Mare's nest "illusory discovery, something of apparent importance causing excitement but which turns out to be a delusion or a hoax" is from 1610s.

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mare (n.2)

"broad, dark area of the moon," 1765, from Latin mare "sea" (from PIE root *mori- "body of water"). Applied to lunar features by Galileo and used thus in 17c. works written in Modern Latin. They originally were thought to be actual seas.

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nightmare (n.)

c. 1300, "an evil female spirit afflicting men (or horses) in their sleep with a feeling of suffocation," compounded from night + mare (n.3) "goblin that causes nightmares, incubus." The meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" is recorded by 1829; that of "very distressing experience" is from 1831. Cognate with Middle Dutch nachtmare, German Nachtmahr. An Old English word for it was niht-genga.

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*mori- 

Proto-Indo-European root meaning "body of water."

It forms all or part of: aquamarine; Armorica; beche-de-mer; cormorant; mare (n.2) "broad, dark areas of the moon;" marina; marinate; marine; mariner; maritime; marsh; mere (n.1) "lake, pool;" Merlin; mermaid; merman; meerschaum; meerkat; morass; Muriel; rosemary; submarine; ultramarine; Weimar.

It is the hypothetical source of/evidence for its existence is provided by: Latin mare; Old Church Slavonic morje, Russian more, Lithuanian marės, Old Irish muir, Welsh mor "sea;" Old English mere "sea, ocean; lake, pool," German Meer "sea."

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marshal (n.)

mid-13c., "high officer of the royal court," charged with regulating ceremonies and maintaining order (early 13c. as a surname), from Old French mareschal "commanding officer of an army; officer in charge of a household" (Modern French maréchal), originally "stable officer, horse tender, groom" (Frankish Latin mariscaluis) from Frankish *marhskalk or a similar Germanic word, literally "horse-servant" (compare Old High German marahscalc "groom," Middle Dutch maerschalc).

This is from a Proto-Germanic compound of *markhaz "horse" (see mare (n.1)) + *skalkaz "servant" (source of Old English scealc "servant, retainer, member of a crew," Dutch schalk "rogue, wag," Gothic skalks "servant"). It corresponds to Old English horsþegn.

From early 14c. as "military commander, general in the army."  In the U.S., a civil officer appointed by the president (with advice and consent of the Senate) in each judicial district as the executive officer of the Supreme Court and the federal courts in his district. For sense development and the tendency of officers of the stable to become chief officers of royal households, compare constable. Also from Germanic are Italian scalco "steward," Spanish mariscal "marshal."

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Whig 

British political party, 1657, in part perhaps a disparaging use of whigg "a country bumpkin" (1640s); but mainly a shortened form of Whiggamore (1649) "one of the adherents of the Presbyterian cause in western Scotland who marched on Edinburgh in 1648 to oppose Charles I." Perhaps originally "a horse drover," from dialectal verb whig "to urge forward" + mare. In 1689 the name was first used in reference to members of the British political party that opposed the Tories. American Revolution sense of "colonist who opposes Crown policies" is from 1768. Later it was applied to opponents of Andrew Jackson (as early as 1825), and taken as the name of a political party (1834) that merged into the Republican Party in 1854-56.

[I]n the spring of 1834 Jackson's opponents adopted the name Whig, traditional term for critics of executive usurpations. James Watson Webb, editor of the New York Courier and Enquirer, encouraged use of the name. [Henry] Clay gave it national currency in a speech on April 14, 1834, likening "the whigs of the present day" to those who had resisted George III, and by summer it was official. [Daniel Walker Howe, "What Hath God Wrought," 2007, p.390]

Whig historian is recorded from 1924. Whig history is "the tendency in many historians ... to emphasise certain principles of progress in the past and to produce a story which is the ratification if not the glorification of the present." [Herbert Butterfield, "The Whig Interpretation of History," 1931]

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sea (n.)

Old English "sheet of water, sea, lake, pool," from Proto-Germanic *saiwa- (source also of Old Saxon seo, Old Frisian se, Middle Dutch see, Swedish sjö), of unknown origin, outside connections "wholly doubtful" [Buck]. Meaning "large quantity" (of anything) is from c. 1200.

Germanic languages also use the general Indo-European word (represented by English mere (n.1)), but have no firm distinction between "sea" and "lake," either by size, by inland or open, or by salt vs. fresh. This may reflect the Baltic geography where the languages are thought to have originated. The two words are used more or less interchangeably in Germanic, and exist in opposite senses (such as Gothic saiws "lake, marshland," marei "sea;" but Dutch zee "sea," meer "lake"). Compare also Old Norse sær "sea," but Danish , usually "lake" but "sea" in phrases. German See is "sea" (fem.) or "lake" (masc.).

Boutkan writes that the sea words in Germanic likely were originally "lake," and the older word for "sea" is represented by haff. The single Old English word glosses Latin mare, aequor, pontus, pelagus, and marmor.

 Meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)). Phrase sea change "transformation," literally "a change wrought by the sea," is attested from 1610, first in Shakespeare ("The Tempest," I.ii). Sea anemone is from 1742; sea legs, humorous colloquial term implying ability to walk on a ship's deck when she is pitching or rolling is from 1712; sea level from 1806; sea urchin from 1590s. At sea in the figurative sense of "perplexed" is attested from 1768, from literal sense of "out of sight of land" (c. 1300).

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*mer- 

Proto-Indo-European root meaning "to rub away, harm." Possibly identical with the root *mer- that means "to die" and forms words referring to death and to beings subject to death.

It forms all or part of: amaranth; ambrosia; amortize; Amritsar; immortal; manticore; marasmus; mare (n.3) "night-goblin, incubus;" morbid; mordacious; mordant; moribund; morsel; mort (n.2) "note sounded on a horn at the death of the quarry;" mortal; mortality; mortar; mortgage; mortify; mortmain; mortuary; murder; murrain; nightmare; post-mortem; remorse.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mrnati "crushes, bruises," mriyate "to kill," martave "to die," mrta- "died, dead," mrtih "death," martah "mortal man," amrta- "immortal;" Avestan miriia- "to die," miryeite "dies," Old Persian martiya- "man;" Hittite mer- "to disappear, vanish," marnu- "to make disappear;" Armenian meranim "to die;" Greek marainein "to consume, exhaust, put out, quench," marasmus "consumption," emorten "died," brotos "mortal" (hence ambrotos "immortal"); Latin mors (genitive mortis) "death," mori "to die;" Armenian merani- "to die;" Gothic maurþr, Old English morþ "murder;" Old Irish marb, Welsh marw "dead;" Lithuanian mirti "to die," mirtis "death;" Old Church Slavonic mreti "to die," mrutvu "dead;" Russian mertvyj, Serbo-Croatian mrtav "dead."

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jade (n.2)
"worn-out horse," late 14c., apparently originally "cart horse," a word of uncertain origin. Barnhart and Century Dictionary suggests a variant of yaid, yald "whore," literally "mare" (c. 1400), from a Scandinavian source akin to Old Norse jalda "mare," and ultimately from Finno-Ugric (compare Mordvin al'd'a "mare"). But OED finds the assumption of a Scandinavian connection "without reason." As a term of abuse for a woman, it dates from 1550s; in early use also of mean or worthless men, and sometimes simply "a young woman."
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