Etymology
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heat (n.)

Old English hætu, hæto "heat, warmth, quality of being hot; fervor, ardor," from Proto-Germanic *haita- "heat" (source also of Old Saxon hittia, Old Norse hiti, Old Frisian hete, German hitze "heat," Gothic heito "fever"), from the same source as Old English hat "hot" and hæða "hot weather" (see hot).

Meaning "a single course in a race," especially a horse race, is from 1660s, perhaps from earlier figurative sense of "violent action; a single intense effort" (late 14c.), or the meaning "run given to a horse to prepare for a race" (1570s). The latter word over time was extended to "division of a race or contest when there are too many contestants to run at once," the winners of each heat then competing in a final race.

Meaning "sexual excitement in animals" is from 1768, especially of females, corresponding to rut in males. Meaning "trouble with the police" attested by 1920. Heat wave "period of excessive hot weather" first attested 1890; earlier in reference to solar cycles. Heat-stroke is from 1858. Heat-seeking (adj.) of missiles, etc., is by 1955. Red heat, white heat are in reference to the color of heated metals, especially iron.

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roaring (adj.)

"that roars or bellows; making or characterized by noise or disturbance," late 14c., present-participle adjective from roar (v.). Used of periods of years characterized by noisy revelry, especially roaring twenties (1930, which OED credits to "postwar buoyancy"); but also, in Australia, roaring fifties (1892, in reference to the New South Wales gold rush of 1851). Roaring Forties in reference to exceptionally rough seas between latitudes 40 and 50 south, is attested from 1841.

The "roaring fifties" are still remembered as the days when Australia held a prosperity never equalled in the world's history and a touch of romance as well. The gold fever never passed away from the land. [E.C. Buley, "Australian Life in Town and Country," 1905]
Roaring boys, roaring lads, swaggerers : ruffians : slang names applied, about the beginning of the seventeenth century, to the noisy, riotous roisterers who infested the taverns and the streets of London, and, in general, acted the part of the Mohocks of a century later. Roaring girls are also alluded to by the old dramatists, though much less frequently. [Century Dictionary]

This is from the use of roar (v.) in old London slang for "behave in a riotous and bullying manner" (1580s).

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yellow (adj.)

Middle English yelwe, from Old English geolu, geolwe, "yellow," from Proto-Germanic *gelwaz (source also of Old Saxon, Old High German gelo, Middle Dutch ghele, Dutch geel, Middle High German gel, German gelb, Old Norse gulr, Swedish gul "yellow"), from PIE root *ghel- (2) "to shine," with derivatives denoting "green" and "yellow" (such as Greek khlōros "greenish-yellow," Latin helvus "yellowish, bay").

In Middle English it also was used of a color closer to blue-gray or gray, of frogs or hazel eyes, and as a translation of Latin caeruleus or glauco. The meaning "light-skinned" (in reference to black persons) is recorded by 1808. It was applied to Asiatics by 1787, though that first reference is to Turkish words for inhabitants of India.

Yellow peril translates German die gelbe gefahr. The sense of "cowardly" is by 1856, of unknown origin; the color was traditionally associated rather with jealousy and envy (17c.). Yellow-bellied "cowardly" is from 1924, probably a semi-rhyming reduplication of yellow; earlier yellow-belly was a sailor's name for a half-caste (1867) and a Texas term for Mexican soldiers (1842, based on the color of their uniforms). Yellow dog "mongrel" is attested from c. 1770; the slang sense of "contemptible person" is recorded by 1881. Yellow fever is attested from 1748, American English (jaundice is a symptom).

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hot (adj.)

Old English hat "hot, flaming, opposite of cold," used of the sun or air, of fire, of objects made hot; also "fervent, fierce, intense, excited," from Proto-Germanic *haita- (source also of Old Saxon and Old Frisian het, Old Norse heitr, Middle Dutch and Dutch heet, German heiß "hot," Gothic heito "heat of a fever"), of uncertain origin, perhaps related to Lithuanian kaisti "to grow hot;" both could be from a substratum word.

With a long vowel in Middle English (rhyming with boat, wrote) which shortened in modern English, perhaps from influence of comparative hotter. As an adverb, Old English hote.

Hot as "full of sexual desire, lustful" is from c. 1500; the sense of "inciting desire" is 18c. Taste sense of "pungent, acrid, biting" is from 1540s. Sense of "exciting, remarkable, very good" is 1895; that of "stolen" is first recorded 1925 (originally with overtones of "easily identified and difficult to dispose of"); that of "radioactive" is from 1942. Of jazz music or combos from 1924.

Hot flashes in the menopausal sense attested from 1887. Hot stuff for anything good or excellent is by 1889, American English. Hot seat is from 1933. Hot potato in figurative sense is from 1846 (from being baked in the fire coals and pulled out hot). Hot cake is from 1680s; to sell like hot cakes is from 1839.

The hot and cold in hide-and-seek or guessing games (19c.) are from hunting (1640s), with notion of tracking a scent. Hot and bothered is by 1921. Hot under the collar in the figurative sense is from 1895.

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scarlet (n.)

mid-13c. (late 12c. as a surname), "rich cloth" (often, but not necessarily, bright red), from a shortened form of Old French escarlate "scarlet (color), top-quality fabric" (12c., Modern French écarlate), which, with Medieval Latin scarlatum "scarlet, cloth of scarlet," Italian scarlatto, Spanish escarlate often is said to be from a Middle Eastern source, but perhaps is rather from a Germanic source akin to Old High German scarlachen, scharlachen (c. 1200), from scar "sheared" + lachen "cloth."

In English it is attested as the name of a color, a highly chromatic and brilliant red, from late 14c. It was used as an adjective in reference to this color, or to gowns of this color, from c. 1300.

Scarlet Lady is Biblical (Isaiah i.18, Revelation xvii.1-5); she has been variously identified by commentators. Scarlet woman "notoriously immoral woman, prostitute" (by 1924) perhaps is from notion of "red with shame or indignation." Earlier it was used in the same sense as Scarlet Lady.

Scarlet fever is from 1670s, so called for its characteristic rash. It also was an old slang term for the condition of women irresistibly glamoured by men in uniform. Scarlet oak, a New World tree, is so called from 1590s. Scarlet letter in figurative use traces to Hawthorne's story (1850), a red cloth "A" which convicted adulterers were condemned to wear. German Scharlach, Dutch scharlaken show influence of words cognate with English lake (n.2).

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spring (n.1)

"season following winter, first of the four seasons of the year; the season in which plants begin to rise," by 1540s, short for spring of the year (1520s), a special sense of an otherwise now-archaic spring (n.) "act or time of springing or appearing; the first appearance; the beginning, birth, rise, or origin" of anything (see spring v., and compare spring (n.2), spring (n.3)). The earliest form seems to have been springing time (late 14c.).

The notion is of the "spring of the year," when plants begin to rise and trees to bud (as in spring of the leaf, 1520s). The Middle English noun also was used of sunrise, the waxing of the moon, rising tides, sprouting of the beard or pubic hair, etc.; compare 14c. spring of dai "sunrise," spring of mone "moonrise." Late Old English spring meant "carbuncle, pustule."

It replaced Old English lencten (see Lent) as the word for the vernal season.  Other Germanic languages tend to take words for "fore" or "early" as their roots for the season name (Danish voraar, Dutch voorjaar, literally "fore-year;" German Frühling, from Middle High German vrueje "early"). In 15c. English, the season also was prime-temps, after Old French prin tans, tamps prim (French printemps, which replaced primevère 16c. as the common word for spring), from Latin tempus primum, literally "first time, first season."

Spring fever is from 1843 as "surge of romantic feelings;" earlier of a type of disease or head-cold prevalent in certain places in spring; Old English had lenctenadle. First record of spring cleaning in the domestic sense is by 1843 (in ancient Persia, the first month, corresponding to March-April, was Adukanaiša, which apparently means "Irrigation-Canal-Cleaning Month;" Kent, p.167). Spring chicken "small roasting chicken" (usually 11 to 14 weeks) is recorded from 1780; transferred sense of "young person" first recorded 1906. Baseball spring training attested by 1889, earlier of militias, etc.

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shake (v.)

Middle English shaken, from Old English sceacan "move (something) quickly to and fro, cause to move with quick vibrations; brandish; move the body or a part of it rapidly back and forth;" also "go, glide, hasten, flee, depart" (as in sceacdom "flight"); also intransitive, of persons or parts of the body, "to tremble" especially from fever, cold, fear (class VI strong verb; past tense scoc, past participle scacen). This is from Proto-Germanic *skakanan (source also of Old Norse, Swedish skaka, Danish skage "to shift, turn, veer"). No certain cognates outside Germanic, but some suggest possible connections to Sanskrit khaj "to agitate, churn, stir about," Old Church Slavonic skoku "a leap, bound," Welsh ysgogi "move."

Of the ground in earthquakes, c. 1300. The meaning "seize and shake" (someone or something else) is from early 14c. From late 14c. in reference to mixing ingredients, etc., by shaking a container. The meaning "weaken, impair" in any respect is from late 14c. on the notion of "make unstable." The meaning "rid oneself of by abrupt twists" is from c. 1200; the modern colloquial use for "get rid of, cast off, abandon" (by 1872, American English) is likely a new extension on the notion of "throw off by a jolting or abrupt action," perhaps with horses in mind. The verb also was used in Middle English as "evade" responsibility, etc.

To shake hands "greet or salute by grasping one another's hands" dates from 1530s. Colloquial shake a (loose) leg "hurry up" is recorded by 1904; to shake a heel (sometimes foot) is an old or provincial way to say "dance" (1660s); to shake (one's) elbow (1620s) meant "to gamble at dice." In 16c.-18c. English, shake (one's) ears was "bestir oneself," an image of animal awakenings. The phrase more _____ than you can shake a stick at "more than you can count" is attested from 1818 (Lancaster, Pa., "Journal"), American English. To shake (one's) head "move one's head from side to side as a sign of disapproval" is recorded from c. 1300.

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war (n.)

late Old English wyrre, werre "large-scale military conflict," from Old North French werre "war" (Old French guerre "difficulty, dispute; hostility; fight, combat, war;" Modern French guerre), from Frankish *werra, from Proto-Germanic *werz-a- (source also of Old Saxon werran, Old High German werran, German verwirren "to confuse, perplex"), from PIE *wers- (1) "to confuse, mix up". Cognates suggest the original sense was "to bring into confusion."

Spanish, Portuguese, and Italian guerra also are from Germanic; Romanic peoples turned to Germanic for a "war" word possibly to avoid Latin bellum (see bellicose) because its form tended to merge with bello- "beautiful." There was no common Germanic word for "war" at the dawn of historical times. Old English had many poetic words for "war" (wig, guð, heaðo, hild, all common in personal names), but the usual one to translate Latin bellum was gewin "struggle, strife" (related to win (v.)).

First record of war-time is late 14c. Warpath (1775) originally is in reference to North American Indians, as are war-whoop (1761), war-paint (1826), and war-dance (1757). War crime is attested from 1906 (in Oppenheim's "International Law"). War chest is attested from 1901; now usually figurative. War games translates German Kriegspiel (see kriegspiel).

The causes of war are always falsely represented ; its honour is dishonest and its glory meretricious, but the challenge to spiritual endurance, the intense sharpening of all the senses, the vitalising consciousness of common peril for a common end, remain to allure those boys and girls who have just reached the age when love and friendship and adventure call more persistently than at any later time. The glamour may be the mere delirium of fever, which as soon as war is over dies out and shows itself for the will-o'-the-wisp that it is, but while it lasts no emotion known to man seems as yet to have quite the compelling power of this enlarged vitality. [Vera Brittain, "Testament of Youth"]
The world will never have lasting peace so long as men reserve for war the finest human qualities. [John Foster Dulles, Speech on the Marshall Plan, 1948]
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