In 1872, Alexander Williams got appointed Captain of the East 35th Street Precinct of the NYPD. At the time, this was a red-light district, brimming with brothels and oozing with potential kickbacks, bribes, and loose money. Mr. Williams wasn't the most scrupulous of gentlemen, and therefore was very happy with his new job. "I've been having chuck steak ever since I've been on the force, and now I'm going to have a bit of tenderloin," he famously remarked. This was a metaphor. Chuck steak is known for being a "subprime" part of a cow, and tenderloin for being very delicious. Here, meat is used to refer to the types of bribes he'll be getting, and Williams is excited that, with all these illegal things going on, he'll be getting the "tenderloin" of all bribes. From there, "tenderloin" evolved into a euphemism for "red light district" in general, with San Francisco adopting a "tenderloin district" of their own for those reasons. The resemblance to the phrase "tender loins" is just a very serendipitous coincidence.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd