Etymology
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fellow (n.)

"companion, comrade," c. 1200, from Old English feolaga "partner, one who shares with another," from Old Norse felagi, from fe "money" (see fee) + lag, from Proto-Germanic *lagam, from PIE root *legh- "to lie down, lay." The etymological sense of fellow seems to be "one who puts down money with another in a joint venture."

Meaning "one of the same kind" is from early 13c.; that of "one of a pair" is from c. 1300. Used familiarly since mid-15c. for "any man, male person," but not etymologically masculine (it is used of women, for example, in Judges xi.37 in the King James version: "And she said unto her father, Let this thing be done for me: let me alone two months, that I may go up and down upon the mountains, and bewail my virginity, I and my fellows").

Its use can be contemptuous or dignified in English and American English, and at different times in its history, depending on who used it to whom, it has carried a tinge of condescension or insult.

University senses (mid-15c., corresponding to Latin socius) evolved from notion of "one of the corporation who constitute a college" and who are paid from its revenues. Fellow well-met "boon companion" is from 1580s, hence hail-fellow-well-met as a figurative phrase for "on intimate terms."

In compounds, with a sense of "co-, joint-," from 16c., and by 19c. also denoting "association with another." Hence fellow-traveler, 1610s in a literal sense but in 20c. with a specific extended sense of "one who sympathizes with the Communist movement but is not a party member" (1936, translating Russian poputchik).

Fellow-countrymen formerly was one of the phrases the British held up to mock the Americans for their ignorance, as it is redundant to say both, until they discovered it dates from the 1580s and was used by Byron and others.

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sensitive (adj.)

late 14c., sensitif, in reference to the body or its parts, "capable of receiving impressions from external objects, having the function of sensation;" also (c. 1400) in scholastic philosophy, "of or pertaining to the faculty of the soul that receives and analyzes sensory information;" from Old French sensitif "capable of feeling" (13c.) and directly from Medieval Latin sensitivus "capable of sensation," from Latin sensus, past participle of sentire "feel, perceive" (see sense (n.)). Also in early Modern English sencitive.

By 1520s as "of, connected with, or affecting the senses." With reference to persons or mental feelings, "keenly susceptible to external influences," especially "easily touched by emotion, readily wounded by unkindness" (but also "ready to take offense"), by 1816.

What is commonly called a 'sensitive' person is one whose sense-organs cannot go on responding as the stimulus increases in strength, but become fatigued. [James Sully, "Outlines of Psychology," 1884]

The mechanical meaning "so delicately adjusted as to respond quickly to very slight changes or conditions" is by 1857. The Cold War meaning "involving national security" is attested by 1953. Related: Sensitively; sensitiveness.

The purely physical sense, in reference to a living being, skin, etc., "having quick or intense response to sensation," is by 1808; it is preserved in sensitive plant (1630s, also in Shelley's poem), a legume which is "mechanically irritable in a higher degree than almost any other plant" [Century Dictionary]. 

Marijuana ... makes you sensitive. Courtesy has a great deal to do with being sensitive. Unfortunately marijuana makes you the kind of sensitive where you insist on everyone listening to the drum solo in Iron Butterfly's 'In-a-Gadda-Da-Vida' fifty or sixty times. [P.J. O'Rourke, "Modern Manners," 1983] 

As a noun, in mesmerism, "one who is sensitive to hypnotic influence," 1850; later "one in whom the sensitive facility is highly developed, an aesthete" (1891).

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hill (n.)

Old English hyll "hill," from Proto-Germanic *hulni- (source also of Middle Dutch hille, Low German hull "hill," Old Norse hallr "stone," Gothic hallus "rock," Old Norse holmr "islet in a bay," Old English holm "rising land, island"), from PIE root *kel- (2) "to be prominent; hill." Formerly including mountains.

In Great Britain heights under 2,000 feet are generally called hills; 'mountain' being confined to the greater elevations of the Lake District, of North Wales, and of the Scottish Highlands; but, in India, ranges of 5,000 and even 10,000 feet are commonly called 'hills,' in contrast with the Himalaya Mountains, many peaks of which rise beyond 20,000 feet. [OED]
The term mountain is very loosely used. It commonly means any unusual elevation. In New England and central New York, elevations of from one to two thousand feet are called hills, but on the plains of Texas, a hill of a few hundred feet is called a mountain. [Ralph S. Tarr, "Elementary Geology," Macmillan, 1903]
Despite the differences in defining mountain systems, Penck (1896), Supan (1911) and Obst (1914) agreed that the distinction between hills, mountains, and mountain systems according to areal extent or height is not a suitable classification. ["Geographic Information Science and Mountain Geomorphology," 2004]

 Figurative phrase over the hill "past one's prime" is recorded by 1950. Expression old as the hills is recorded by 1819, perhaps echoing Job xv.7. Earlier form old as the hills and the valleys is attested by 1808:

And this is no "new morality." It is morality as old as the hills and the valleys. It is a morality which must be adopted; or, we must confess that there are certain political evils greater than that of seeing one's country conquered. [Cobbett's Weekly Political Register, Feb. 6, 1808]

Cobbett's also had, on April 11, 1818:

However, thus it always is: "those whom God intends to destroy, he first makes foolish," which is a saying as old as the hills between Everly and Marlborough.
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slang (n.)

1756, "special vocabulary of tramps or thieves," later "jargon of a particular profession" (1801). The sense of "very informal language characterized by vividness and novelty" is by 1818.

Anatoly Liberman writes here an extensive account of the established origin of the word from the Northern England noun slang "a narrow piece of land running up between other and larger divisions of ground" and the verb slanger "linger, go slowly," which is of Scandinavian origin (compare Norwegian slenge "hang loose, sling, sway, dangle," Danish slænge "to throw, sling"). "Their common denominator seems to be 'to move freely in any direction' " [Liberman]. Noun derivatives of these (Danish slænget, Norwegian slenget) mean "a gang, a band," and Liberman compares Old Norse slangi "tramp" and slangr "going astray" (used of sheep). He writes:

It is not uncommon to associate the place designated for a certain group and those who live there with that group’s language. John Fielding and the early writers who knew the noun slang used the phrase slang patter, as though that patter were a kind of talk belonging to some territory.

So the sense evolution would be from slang "a piece of delimited territory" to "the territory used by tramps for their wandering," to "their camping ground," and finally to "the language used there." The sense shift then passes through itinerant merchants:

Hawkers use a special vocabulary and a special intonation when advertising their wares (think of modern auctioneers), and many disparaging, derisive names characterize their speech; charlatan and quack are among them.

Liberman concludes: 

[Slang] is a dialectal word that reached London from the north and for a long time retained the traces of its low origin. The route was from "territory; turf" to "those who advertise and sell their wares on such a territory," to "the patter used in advertising the wares," and to "vulgar language" (later to “any colorful, informal way of expression”).

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[S]lang is a conscious offence against some conventional standard of propriety. A mere vulgarism is not slang, except when it is purposely adopted, and acquires an artificial currency, among some class of persons to whom it is not native. The other distinctive feature of slang is that it is neither part of the ordinary language, nor an attempt to supply its deficiencies. The slang word is a deliberate substitute for a word of the vernacular, just as the characters of a cipher are substitutes for the letters of the alphabet, or as a nickname is a substitute for a personal name. [Henry Bradley, from "Slang," in Encyclopedia Britannica, 11th ed.]

A word that ought to have survived is slangwhanger (1807, American English) "noisy or abusive talker or writer."

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horse (n.)

"solidungulate perissodactyl mammal of the family Equidæ and genus Equus" [Century Dictionary], Old English hors "horse," from Proto-Germanic *harss- (source also of Old Norse hross, Old Frisian, Old Saxon hors, Middle Dutch ors, Dutch ros, Old High German hros, German Roß "horse"), of unknown origin. By some, connected to PIE root *kers- "to run," source of Latin currere "to run." Boutkan prefers the theory that it is a loan-word from an Iranian language (Sarmatian) also borrowed into Uralic (compare Finnish varsa "foal"),

The usual Indo-European word is represented by Old English eoh, Greek hippos, Latin equus, from PIE root *ekwo-. Another Germanic "horse" word is Old English vicg, from Proto-Germanic *wegja- (source also of Old Frisian wegk-, Old Saxon wigg, Old Norse vigg), which is of uncertain origin. In many other languages, as in English, this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in Indo-European religion. For the Romanic words (French cheval, Spanish caballo) see cavalier (n.); for Dutch paard, German Pferd, see palfrey; for Swedish häst, Danish hest see henchman. As plural Old English had collective singular horse as well as horses, in Middle English also sometimes horsen, but horses has been the usual plural since 17c.

Used at least since late 14c. of various devices or appliances which suggest a horse (as in sawhorse), typically in reference to being "that upon which something is mounted." For sense of "large, coarse," see horseradish. Slang use for "heroin" is attested by 1950. To ride a horse that was foaled of an acorn (1670s) was through early 19c. a way to say "be hanged from the gallows." Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Horse-pistol, "large one-handed pistol used by horseback riders," is by 1704. A dead horse as a figure for something that has ceased to be useful is from 1630s; to flog a dead horse "attempt to revive interest in a worn-out topic" is from 1864.

HORSEGODMOTHER, a large masculine wench; one whom it is difficult to rank among the purest and gentlest portion of the community. [John Trotter Brockett, "A Glossary of North Country Words," 1829]

The term itself is attested from 1560s. The horse's mouth as a source of reliable information is from 1921, perhaps originally of racetrack tips, from the fact that a horse's age can be determined accurately by looking at its teeth. To swap horses while crossing the river (a bad idea) is from the American Civil War and appears to have been originally one of Abe Lincoln's stories. Horse-and-buggy meaning "old-fashioned" is recorded from 1926 slang, originally in reference to a "young lady out of date, with long hair." To hold (one's) horses "restrain one's enthusiasm, be patient" is from 1842, American English; the notion is of keeping a tight grip on the reins.

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fuck (v.)

"to have sexual intercourse with" (transitive), until recently a difficult word to trace in usage, in part because it was omitted as taboo by the editors of the original OED when the "F" entries were compiled (1893-97). Johnson also had excluded the word, and fuck wasn't in a single English language dictionary from 1795 to 1965. "The Penguin Dictionary" broke the taboo in the latter year. Houghton Mifflin followed, in 1969, with "The American Heritage Dictionary," but it also published a "Clean Green" edition without the word, to assure itself access to the public high school market.

Written form attested from at least early 16c.; OED 2nd edition cites 1503, in the form fukkit, and the earliest attested appearance of current spelling is 1535 ("Bischops ... may fuck thair fill and be vnmaryit" [Sir David Lyndesay, "Ane Satyre of the Thrie Estaits"]). Presumably it is a more ancient word, but one not written in the kind of texts that have survived from Old English and Middle English [September 2015: the verb appears to have been found recently in an English court manuscript from 1310]. Buck cites proper name John le Fucker from 1278, but that surname could have other explanations. The word apparently is hinted at in a scurrilous 15c. poem, titled "Flen flyys" ["Fleas, Flies (and Friars)"], written in bastard Latin and Middle English. The relevant line reads:

Non sunt in celi
quia fuccant uuiuys of heli

"They [the monks] are not in heaven because they fuck the wives of [the town of] Ely." Fuccant is pseudo-Latin, and in the original it is written in cipher. The earliest examples of the word otherwise are from Scottish, which suggests a Scandinavian origin, perhaps from a word akin to Norwegian dialectal fukka "copulate," or Swedish dialectal focka "copulate, strike, push," and fock "penis."

Another theory traces the Modern English verb to Middle English fyke, fike "move restlessly, fidget" (see fike) which also meant "dally, flirt," and probably is from a general North Sea Germanic word (compare Middle Dutch fokken, German ficken "fuck," earlier "make quick movements to and fro, flick," still earlier "itch, scratch;" the vulgar sense attested from 16c.). This would parallel in sense the vulgar Middle English term for "have sexual intercourse," swive, from Old English swifan "to move lightly over, sweep" (see swivel). But OED remarks that these "cannot be shown to be related" to the English word. Liberman has this to say:

Germanic words of similar form (f + vowel + consonant) and meaning 'copulate' are numerous. One of them is G. ficken. They often have additional senses, especially 'cheat,' but their basic meaning is 'move back and forth.' ... Most probably, fuck is a borrowing from Low German and has no cognates outside Germanic.

Chronology and phonology rule out Shipley's attempt to derive it from Middle English firk "to press hard, beat." The unkillable urban legend that this word is an acronym of some sort (a fiction traceable on the internet to 1995 but probably predating that), and the "pluck yew" fable, are results of ingenious trifling (also see here). The Old English verb for "have sexual intercourse with" was hæman, from ham "dwelling, home," with a sense of "take home, co-habit." French foutre and Italian fottere seem to resemble the English word but are unrelated, descending rather from Latin futuere, which perhaps is from PIE root *bhau- "to strike," extended via a figurative use "from the sexual application of violent action" [Shipley; compare the sexual slang use of bang, etc.].

Fuck was outlawed in print in England (by the Obscene Publications Act, 1857) and the U.S. (by the Comstock Act, 1873). The word continued in common speech, however. During World War I:

It became so common that an effective way for the soldier to express this emotion was to omit this word. Thus if a sergeant said, 'Get your ----ing rifles!' it was understood as a matter of routine. But if he said 'Get your rifles!' there was an immediate implication of urgency and danger. [John Brophy, "Songs and Slang of the British Soldier: 1914-1918," pub. 1930]

The legal barriers against use in print broke down in mid-20c. with the "Ulysses" decision (U.S., 1933) and "Lady Chatterley's Lover" (U.S., 1959; U.K., 1960). The major breakthrough in publication was James Jones' "From Here to Eternity" (1950), with 50 fucks (down from 258 in the original manuscript).

The abbreviation F (or eff) probably began as euphemistic, but by 1943 it was regarded as a cuss word in its own right. In 1948, the publishers of "The Naked and the Dead" persuaded Norman Mailer to use the euphemism fug. When Mailer later was introduced to Dorothy Parker, she greeted him with, "So you're the man who can't spell 'fuck' " [The quip sometimes is attributed to Tallulah Bankhead]. As a written euphemism, muck is attested by 1929 (Hemingway used it in "For Whom the Bell Tolls," 1940). Related: Fucked; fucking.

Fuck-all "nothing" first recorded 1960. Verbal phrase fuck up "to ruin, spoil, destroy" is attested from c. 1916. A widespread group of Slavic words (such as Polish pierdolić) can mean both "fornicate" and "make a mistake." Fuck off is attested from 1929; as a command to depart, by 1944. Egyptian legal agreements from the 23rd Dynasty (749-21 B.C.E.) frequently include the phrase, "If you do not obey this decree, may a donkey copulate with you!" [Reinhold Aman, "Maledicta," Summer 1977].

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