Jew (n.)
late 12c. (in plural, giwis), from Anglo-French iuw, Old French giu, from Latin Iudaeum (nominative Iudaeus), from Greek Ioudaios, from Aramaic jehudhai (Hebrew y'hudi) "Jew," from Y'hudah "Judah," literally "celebrated," name of Jacob's fourth son and of the tribe descended from him. Replaced Old English 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 German Judenhetze. In uneducated times, inexplicable ancient artifacts were credited to Jews, based on the biblical chronology of history: such as 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.
jew (v.)
"to cheat, to drive a hard bargain," 1824, from Jew (n.) (compare gyp, welsh, etc.). The campaign to eliminate it in early 20c. was so successful that people began to avoid the noun and adjective, too, and started using Hebrew instead.
Now I'll say 'a Jew' and just the word Jew sounds like a dirty word and people don't know whether to laugh or not. [Lenny Bruce (1925-1966)]