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Jew (n.)
late 12c. (in plural, giwis), from Anglo-Fr. iuw, O.Fr. giu, from L. Judaeum (nom. Judaeus), from Gk. Ioudaios, from Aramaic jehudhai (Heb. y'hudi) "Jew," from Y'hudah "Judah," lit. "celebrated," name of Jacob's fourth son and of the tribe descended from him. Replaced O.E. Iudeas "the Jews." Originally, "Hebrew of the kingdom of Judah." Jews' harp "simple mouth harp" is from 1580s, earlier Jews' trump (1540s); the connection with Jewishness is obscure. Jew-baiting first recorded 1853, in reference to Ger. Judenhetze. In uneducated times, inexplicable ancient artifacts were credited to Jews, based on the biblical chronology of history: e.g. Jews' money (1570s) "Roman coins found in England." In Greece, after Christianity had erased the memory of classical glory, ruins of pagan temples were called "Jews' castles," and in Cornwall, Jews' houses was the name for the remains of ancient tin-smelting works.
http://www.etymonline.com/index.php?allowed_in_frame=0&search=jew&searchmode=none

какое-то мутное объяснение - якобы средневековое невежество и влияние библейской хронологии - вынуждали называть "древне"-римские монеты "еврейскими деньгами", а руины "языческих" храмов в Греции "еврейскими замками". Уж не считали ли они древних римлян и греков древними евреями (а древних евреев древними римлянами и греками)?
оловоплавление; сходным занятием, лужением, занимались цыгане, правда возможно в более поздние времена; в Корнуолле же были важные месторождения олова в средние века