early 13c., signe, "gesture or motion of the hand," especially one meant to express thought or convey an idea, from Old French signe "sign, mark," from Latin signum "identifying mark, token, indication, symbol; proof; military standard, ensign; a signal, an omen; sign in the heavens, constellation."

According to Watkins, literally "standard that one follows," from PIE *sekw-no-, from root *sekw- (1) "to follow." De Vaan has it from PIE *sekh-no- "cut," from PIE root *sek- "to cut" He writes: "The etymological appurtenance to seco 'to cut' implies a semantic shift of *sek-no- 'what is cut out', 'carved out' > 'sign'." But he also also compares Hebrew sakkin, Aramaic sakkin "slaughtering-knife," and mentions a theory that "both words are probably borrowed from an unknown third source."

It has ousted native token. By c. 1300 as "an indication of some coming event." The meaning "a visible mark or device having some special meaning" is recorded from late 13c.; that of "miraculous manifestation, a miracle demonstrating divine power" is from c. 1300. In reference to one of the 12 divisions of the zodiac, from mid-14c.

The sense of "inscribed board with a characteristic device attached to the front of an inn, shop, etc.," to distinguish it from others is recorded from mid-15c. The meaning "indicator, token or signal of some condition" (late 13c.) is behind sign of the times (1520s). The meaning "conventional mark or symbol in place of words" (in music, mathematics, etc., as in plus sign) is by 1550s. In some uses, the word probably is a shortening of ensign. 

late 14c., in reference to the grammatical case, from Old French genitif or directly from Latin (casus) genitivus "case expressing possession, source, or origin," from genitivus "of or belonging to birth," from genitus, past participle of gignere "to beget, produce" (from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups).

This word was misused by Latin grammarians to render Greek genikē (ptōsis) "the general or generic (case)," expressing race or kind (Greek genikos "belonging to the family"), from genos "family, race, birth, descent," from the same PIE root as the Latin word. As the genitive also is the case of the possessor, and the Romans "were not strong in abstract matters" [Gilbert Murray], the result was some confusion.

The Latin genitivus is a mere blunder, for the Greek word genike could never mean genitivus. Genitivus, if it is meant to express the case of origin or birth, would in Greek have been called gennetike, not genike. Nor does the genitive express the relation of son to father. For though we may say, "the son of the father," we may likewise say, "the father of the son." Genike, in Greek, had a much wider, a much more philosophical meaning. It meant casus generalis, the general case, or rather the case which expresses the genus or kind. This is the real power of the genitive. If I say, "a bird of the water," "of the water" defines the genus to which a certain bird belongs; it refers to the genus of water-birds. [Max Müller, "Lectures on the Science of Language," 1861]

 The noun meaning "the genitive case in grammar" is from 1610s.

late 14c., nerve, nerf, "sinew, tendon, hard cord of the body" (a sense now obsolete), also "fiber or bundle of fibers that convey the capacity to feel or move from the brain or spinal cord to the body," from Old French nerf and directly from Medieval Latin nervus "a nerve," from Latin nervus "sinew, tendon; cord, bowstring, string of a musical instrument," metathesis of pre-Latin *neuros, from PIE *(s)neu- "tendon, sinew" (source also of Sanskrit snavan- "band, sinew," Armenian neard "sinew," Greek neuron "sinew, tendon," in Galen "nerve").

The late medieval surgeons understood the nature and function of the nerves and often used nervus to denote a `nerve' in the modern sense, as well as to denote a `tendon'. There appears to have been some confusion, however, between nerves and tendons; hence, a number of instances in which nervus may be interpreted in either way or in both ways simultaneously. [Middle English Compendium] 

The secondary senses developed from meaning "strength, vigor; force, energy" (c. 1600), from the "sinew" sense. Hence the non-scientific sense with reference to feeling or courage, first attested c. 1600 (as in nerves of steel, 1869) and that of "coolness in the face of danger, fortitude under trying or critical circumstances" is by 1809. The bad sense "impudence, boldness, cheek" (originally slang) is by 1887. Latin nervus also had a figurative sense of "vigor, force, power, strength," as did Greek neuron. From the neurological sense come Nerves "condition of hysterical nervousness," attested by 1890, perhaps from 1792. To get on (someone's) nerves is from 1895. War of nerves "psychological warfare" is from 1915.

Old English horn "horn of an animal; projection, pinnacle," also "wind instrument" (originally one made from animal horns), from Proto-Germanic *hurni- (source also of German Horn, Dutch horen, Old Frisian horn, Gothic haurn), from PIE root *ker- (1) "horn; head."

Late 14c. as "one of the tips of the crescent moon." The name was retained for a class of musical instruments that developed from the hunting horn; the French horn is the true representative of the class. Of dilemmas from 1540s; of automobile warning signals from 1901. Slang meaning "erect penis" is suggested by c. 1600. Jazz slang sense of "trumpet" is by 1921. Meaning "telephone" is by 1945. Figurative senses of Latin cornu included "salient point, chief argument; wing, flank; power, courage, strength." Horn of plenty is from 1580s. To make horns at "hold up the fist with the two exterior fingers extended" as a gesture of insult is from c.1600.

Symbolic of cuckoldry since mid-15c. (the victim was fancied to grow one on his head). The image is widespread in Europe and perhaps as old as ancient Greece. The German linguist Hermann Dunger ('Hörner Aufsetzen' und 'Hahnrei', "Germania" 29, 1884) ascribes it to a custom surviving into 19c., "the old practice of engrafting the spurs of a castrated cock on the root of the excised comb, which caused them to grow like horns" [James Hastings, "Encyclopedia of Religion and Ethics"] but the image could have grown as well from a general gesture of contempt or insult made to wronged husbands, "who have been the subject of popular jest in all ages" [Hastings].

c. 1200, "principle or maxim governing conduct, formula to which conduct must be conformed" from Old French riule, Norman reule "rule, custom, (religious) order" (in Modern French partially re-Latinized as règle), from Vulgar Latin *regula, from Latin regula "straight stick, bar, ruler;" figuratively "a pattern, a model," related to regere "to rule, straighten, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

By mid-14c. as "control, government, sway, dominion." The meaning "regulation governing play of a game, etc." is from 1690s; the phrase rules of the game is by 1787. To bend the rules "interpret leniently, overlook infringement" is by 1680s.

The meaning "strip with a straight edge used for making straight lines or measuring" is from mid-14c. Typography sense of "thin strip cut type-high and used for printing continuous lines" is attested from 1680s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Rule of the road in reference to the fixed customs, formerly much varying from country to country, which regulate the sides to be taken by vehicles in passing each other, is by 1805.

The rule of the road is a paradox quite,

In driving your carriage along,

If you keep to the left you are sure to go right,

If you keep to the right you go wrong.

[Horne Tooke, "Diversions of Purley," 1805]

Middle English shadwe, from Old English sceadwe, sceaduwe "shade, the effect of interception of sunlight; dark image cast by someone or something when interposed between an object and a source of light," oblique cases ("to the," "from the," "of the," "in the") of sceadu (see shade (n.)). Shadow is to shade (n.) as meadow is to mead (n.2). Similar formation in Old Saxon skado, Middle Dutch schaeduwe, Dutch schaduw, Old High German scato, German schatten, Gothic skadus "shadow, shade."

From mid-13c. as "darkened area created by shadows, shade." From early 13c. in sense "anything unreal;" mid-14c. as "a ghost." Many senses are from the notion of "that which follows or attends a person." From late 14c. as "a foreshadowing, prefiguration." Meaning "imitation, copy" is from 1690s. Sense of "the faintest trace" is from 1580s; that of "a spy who follows" is from 1859. Many of the modern English senses also were in Latin umbra, Greek skia, along with that of "uninvited guest who an invited one brings with him."

As a designation of members of an opposition party chosen as counterparts of the government in power, it is recorded from 1906. Shadow of Death (c. 1200) translates Vulgate umbra mortis (Psalms xxiii.4, etc.), which translates Greek skia thanatou, itself perhaps a mistranslation of a Hebrew word for "intense darkness." In "Beowulf," Grendel is a sceadugenga, a shadow-goer, and another word for "darkness" is sceaduhelm. To be afraid of one's (own) shadow "be very timorous" is from 1580s.

"Native North American shaman," by 1801, from adoption of the word medicine in native speech with a sense of "magical influence; something supposed to possess curative, supernatural, or mysterious power." The U.S.-Canadian boundary they called the Medicine Line (attested by 1880), because it conferred a kind of magic protection: punishment for crimes committed on one side of it could be avoided by crossing over to the other. Compare Middle English use of medicine in secondary senses of "moral, psychological, or social remedy; safeguard, defense."

Unless some understanding is arrived at between the American and Canadian Governments that offenders may be promptly and vigorously dealt with, I very much fear that killing and stealing will increase to such an extent that the country along the border will be scarcely habitable. When the Indians are made to understand that the mere fact of "hopping" across the line does not exempt them from punishment, there will be a much greater guarantee of their good behaviour. Now they call the boundary the "Medicine line," because no matter what they have done upon one side they feel perfectly secure after having arrived upon the other. [Report of Superintendent L.N.F. Crozier, Dec. 1880, in "North-West Mounted Police Force Commissioner's Report," 1880]

Hence also medicine bag "pouch containing some article supposed to possess curative or magical powers, worn on the person by native North American people" (1802). 

late 15c. (earlier ethnical, early 15c.) "pagan, heathen," from Late Latin ethnicus, from Greek ethnikos "of or for a nation, national," by some writers (Polybius, etc.) "adopted to the genius or customs of a people, peculiar to a people," and among the grammarians "suited to the manners or language of foreigners," from ethnos "band of people living together, nation, people, tribe, caste," also used of swarms or flocks of animals, properly "people of one's own kind," from PIE *swedh-no-, suffixed form of root *s(w)e-, third person pronoun and reflexive, also forming words referring to the social group (see idiom). Earlier in English as a noun, "a heathen, pagan, one who is not a Christian or Jew" (c. 1400). In modern noun use, "member of an ethnic group," from 1945.

In Septuagint, Greek ta ethne translates Hebrew goyim, plural of goy "nation," especially of non-Israelites, hence especially "gentile nation, foreign nation not worshipping the true God" (see goy), and ethnikos is used by ecclesiastical writers in a sense of "savoring of the nature of pagans, alien to the worship of the true God," and as a noun "the pagan, the gentile." The classical sense of "peculiar to a race or nation" in English is attested from 1851, a return to the word's original meaning; that of "different cultural groups" is 1935; and that of "racial, cultural or national minority group" is American English 1945. Ethnic cleansing is attested from 1991.

Although the term 'ethnic cleansing' has come into English usage only recently, its verbal correlates in Czech, French, German, and Polish go back much further. [Jerry Z. Muller, "Us and Them: The Enduring Power of Ethnic Nationalism," Foreign Affairs, March/April 2008]

also bellydance, 1883, in a British account of travels in Persia, from belly (n.) + dance (n.). In early use sometimes referred to by the French danse du ventre, which is attested by 1872 in French accounts from the Middle East. It appears as a French term in English by 1883, and its use got a boost from the performances of it at the Paris Exposition of 1889.

We agreed, and made our way to the mimic street called Grand Cairo, where we witnessed the lady contortionist who performs a series of movements, designated with charming frankness on the affiches as "La Danse du Ventre." It might with equal candor be called the Lumbar Wriggle [or] the Pectoral Squirm, for this curious Arab almeh possesses the power of moving any one of her principal sets of muscles quite independently of all the others, and can make any prominent part of her person waggle or surge, while its neighboring lines or curves preserve a statuesque rigidity. [Table Talk, September 1889]

The number of women [in the audience] was ludicrously disproportionate, and the number of American women was noticeable. Some of them seemed slightly pensive, but all were interested. Their large eyes grew larger still. They almost forgot decorum in crowding for a better view, in leaning over the backs of chairs in concentrated absorbed attention. [Scribner's Magazine, January 1890]

The English noun is perhaps a direct translation of the French. As a verb from 1963. Related: Belly-dancer (1922); belly-dancing (n.), 1921.