former district in New York City, in which there was much graft and corruption: so called because regarded as a choice assignment for police seeking graft
See tenderloin in American Heritage Dictionary 4
noun
The tenderest part of a loin of beef, pork, or similar cut of meat.
A city district notorious for vice and graft.
Origin:
Origin: Sense 2, after the Tenderloin, an area of New York City (from the easy income it once afforded corrupt policemen)