In the Western world, Jews are stereotyped as being pennypinching as a whole; in China, people from Shanghai are said to be willing to do anything to save themselves a few yuan. Thus, a Shanghai Jew would be a person so miserly as to be beyond belief.
"I told my landlord the heat wasn't working right. You know what the little Shanghai Jew did? Came in, turned off the radiator, handed me one of those little ceramic heaters, & left. That was 2 weeks ago.