Etymology
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hair (n.)

Old English hær "hair, a hair," from Proto-Germanic *hēran (source also of Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (source also of Lithuanian šerys "bristle;" see horror).

Spelling influenced by Old Norse har and Old English haire "haircloth," from Old French haire, from Frankish *harja or some other Germanic source (see above). Hair-dye is from 1803. To let one's hair down "become familiar" is first recorded 1850. Homeopathic phrase hair of the dog (that bit you), remedy from the same thing that caused the malady, especially a drink on the morning after a debauch, 1540s in English, is in Pliny.

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oil (n.)

late 12c., "olive oil," from Anglo-French and Old North French olie, from Old French oile, uile "oil" (12c., Modern French huile), from Latin oleum "oil, olive oil" (source of Spanish, Italian olio), from Greek elaion "olive tree," from elaia (see olive).

Nearly all the European languages' words for "oil" (Croatian ulje, Polish olej, Hungarian olaj, Albanian uli, Lithuanian alejus, etc.) are from the Greek word; the Germanic (except Gothic) and Celtic one coming from Greek via Latin: Old English æle, Dutch olie, German Öl, Welsh olew, Gaelic uill, etc. 

In English it meant "olive oil" exclusively till c. 1300, when the word began to be extended to any fatty, greasy liquid substance (usually flammable and insoluble in water). Often especially "oil as burned in a lamp to afford light" (as in midnight oil, symbolizing late work). Use for "petroleum" is recorded from 1520s but not common until 19c.

The artist's oils (1660s), short for oil-color (1530s), are paints made by grinding pigment in oil; oil-painting "art or craft of painting in oils" is by 1690s.  The ocean-going oil-tanker is from 1900; oil-spill in the environmental catastrophe sense is by 1924. As a condiment, oil and vinegar is attested from 1620s. The figurative expression pour oil upon the waters "appease strife or disturbance" is by 1840, from an ancient trick of sailors.

Another historical illustration which involves monolayers, was when sailors poured oil on the sea in order to calm 'troubled waters' and so protect their ship. This worked by wave damping or, more precisely, by preventing small ripples from forming in the first place so that the wind could have no effect on them. [J. Lyklema, "Fundamentals of Interface and Colloid Science," Academic Press, 2000]

The phenomenon depends on what are called Marangoni effects; Benjamin Franklin experimented with it in 1765.

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oil (v.)

"to smear or rub with oil or ointment," mid-15c., oilen, from oil (n.). Later especially "to lubricate (machinery)." Related: Oiled; oiling. An Old English verb in this sense was besmyrian.

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hair-trigger (n.)

1795; Figurative use by 1841. Hair perhaps in reference to the slight pressure required to activate it.

The difference between a hair-trigger and a common trigger is this—the hair-trigger, when set, lets off the cock by the slightest touch, whereas the common trigger requires a considerable degree of force, and consequently is longer in its operation. [Charles James, "Military Dictionary," London, 1802]
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oil-skin (n.)

also oilskin, "cloth of cotton, linen, etc., prepared with oil to make it water-proof," by 1714, from oil (n.) + skin (n.).

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long-hair (n.)

also longhair, 1893, "cat with long hair," from long (adj.) + hair (n.). As "intellectual," especially in musical tastes, "devotee of classical music," 1920 (late 19c. long hair was noted as a characteristic of classical musicians, perhaps inspired by the famous locks of Liszt). Sense of "hippie" attested from 1969. The adjective long-haired is attested from mid-15c.

Forty years ago, a music teacher who was not born abroad and who did not wear long hair was regarded with suspicion. He was spurious—not the real thing. On the face of it he could not be a good musician. [W. Francis Gates, in "The Music Student," vol. i, no. 3, October 1915]
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cross-hair (n.)

also crosshair, cross-hairs, "very fine line (originally spider's silk) stretched across the focal plane of a telescope or microscope, forming a cross with another," 1755 of a telescope, 1780 in gunnery, from cross- + hair (n.). Also often in early 19c. spider-line, spider's-line (1819).

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hair-splitting (n.)

"making over-nice distinctions," by 1739, from hair + verbal noun from split (v.). To split hairs "make over-fine distinctions" is first recorded 1650s, as to cut the hair. Hair also being 18c. slang for "female pudendum," hair-splitter was noted in 1811 as slang for "penis."

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hair-shirt (n.)

garment of ascetics and penitents, 1680s, from hair + shirt. Figurative use by 1884. Earlier, such a garment was called simply a hair (c. 1200); and compare haircloth.

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short-hair (n.)

type of domestic cat, by 1890, abbreviated from short-hair cat, from short (adj.) + hair (n.).

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