Etymology
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jar (v.)
1520s, "to make a brief, harsh, grating sound," often in reference to bird screeches; the word often is said to be echoic or imitative; compare jargon (n.), jay (n.), garrulous. Figurative sense of "have an unpleasant effect on" is from 1530s; that of "cause to vibrate or shake" is from 1560s. Related: Jarred; jarring. As a noun in this sense from 1540s.
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Leyden 
modern Leiden, city in Holland, said to be from Germanic *leitha- "canal." Leyden jar, phial used for accumulating and storing static electricity (1755), so called because it was first described (in 1746) by physicist Pieter van Musschenbroek (1692-1761) of Leyden.
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jar (n.)

"simple earthen or glass cylindrical vessel," early 15c., possibly from rare Old French jarre "liquid measure smaller than a barrel," or more likely from Medieval Latin jarra (13c.) or Spanish or Catalan jarra (13c.), all ultimately from Arabic jarrah "earthen water vessel, ewer" (whence also Provençal jarra, Italian giarra), a general word in the 13c. Mediterranean sea-trade, which is from Persian jarrah "a jar, earthen water-vessel." Originally in English a large container used for importing olive oil.

In Britain in the 15th to 17th centuries, oil-lamps were overall not often used, because the oil was too expensive. Usage increased in the 17th century despite the expense. Olive oil was the most-often-used type of oil in the oil-lamps until the 18th century. The indications are good that no country or region exported more oil to Britain than southern Spain did in the 15th-17th centuries, with southern Italy coming second. ["English Words of Arabic Ancestry"]
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bell-jar (n.)
1830, so called for its shape, from bell (n.) + jar (n.). Earlier was bell-glass (1680s).
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Indo-China 
also Indochina, "Farther India, the region between India and China," 1815, from Indo- "India" + China. The name was said to have been proposed by Scottish poet and orientalist John Leyden, who lived and worked in India from 1803 till his death at 35 in 1811. French Indo-Chine is attested from 1813, but the source credits it to Leyden. The inappropriateness of the name was noticed from the start. Related: Indo-Chinese (1814).
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jarhead (n.)
also jar-head, "U.S. Marine," by 1985 (but in a biographical book with a World War II setting), from jar + head (n.). Also used as a general term of insult (by 1979) and by 1922 as a Georgia dialectal word for "mule."
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twist-off (adj.)
of bottle or jar caps, 1959, from the verbal phrase; see twist (v.) + off (adv.).
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jarring (adj.)
"having a sharp, unpleasant effect," 1550s, present-participle adjective from jar (v.). Related: Jarringly.
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gill (n.2)

liquid measure (in modern use commonly a quarter of a pint), late 13c., from Old French gille, a wine measure, and from Medieval Latin gillo "earthenware jar," words of uncertain origin, perhaps related to the source of gallon.

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ajar (adv.)
"slightly open, neither open nor shut," 1718, also on a jar, on the jar, perhaps from Scottish dialectal a char "turned a little way," earlier on char (mid-15c.) "on the turn (of a door or gate)," from Middle English char "a turn," from Old English cier "a turn" (see chore). For first element see a- (1). For unusual change of ch- to j-, compare jowl.
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