Etymology
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Castor 

one of the divine twins (brother of Pollux), also the name of the alpha star of Gemini, Latin, from Greek Kastor, perhaps literally "he who excels." They were sons of Tyndarus, king of Sparta (but in post-Homeric myth of Zeus in the form of a swan)  and Leda.

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castor (n.)

late 14c., "a beaver," from Old French castor (13c.), from Latin castor "beaver," from Greek kastor "beaver," perhaps literally "he who excels," and thus identical with the name of one of the divine twins (with Pollux), worshipped by women in ancient Greece as a healer and preserver from disease (see Castor).

It has been assumed that the hero's name was given to the animal because he was a noted healer and the odorous reddish-brown secretions of the inguinal sacs of the animal (Latin castoreum), were used medicinally in ancient times, especially for women's diseases. But the animal did not live in Greece in classical times (the closest beavers were north of the Black Sea), and the name probably was borrowed from another language, perhaps influenced by the hero's name. The Greek word replaced the native Latin word for "beaver" (fiber).

In English, castor is attested in the secretion sense from late 14c. Modern castor oil is so-called by 1746; it is made from seeds of the plant Ricinus communis but supposedly possesses the laxative qualities (and taste) of beaver juice.

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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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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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oil-cloth (n.)

also oilcloth, 1690s, "cotton or a similar fabric waterproofed with oil," from oil (n.) + cloth. In reference to an oil-treated canvas used as a cheap floor covering, 1796.

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oil-well (n.)

"a boring in the earth made for petroleum," 1847, from oil (n.) + well (n.).

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oil-can (n.)

"can for holding oil," especially one with a long, narrow, tapering spout, used to oil machinery, 1839, from oil (n.) + can (n.).

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oil-tank (n.)

"tank for holding oil," 1862, from oil (n.) + tank (n.).

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oil-mill (n.)

"a crushing- or grinding-machine for expressing oil from seeds, fruits, nuts, etc.," early 15c., from oil (n.) + mill (n.1).

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