"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"]
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.