The Collaborative International Dictionary of English v.0.48:

Tenderloin \Ten"der*loin`\, n.
   1. A strip of tender flesh on either side of the vertebral
      column under the short ribs, in the hind quarter of beef
      and pork. It consists of the psoas muscles.
      [1913 Webster]

   2. In New York City, the region which is the center of the
      night life of fashionable amusement, including the
      majority of the theaters, etc., centering on Broadway. The
      term orig. designates the old twenty-ninth police
      precinct, in this region, which afforded the police great
      opportunities for profit through conniving at vice and
      lawbreaking, one captain being reported to have said on
      being transferred there that whereas he had been eating
      chuck steak he would now eat tenderlion. Hence, in some
      other cities, a district largely devoted to night
      amusement, or, sometimes, to vice.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tenderloin
    n 1: a city district known for its vice and high crime rate
         [syn: combat zone, tenderloin]
    2: the tender meat of the loin muscle on each side of the
       vertebral column [syn: tenderloin, undercut]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "tenderloin":
   Bowery, Chateaubriand, Chinatown, East End, East Side,
   Little Hungary, Little Italy, West End, West Side, bagnio, barrio,
   bawdyhouse, black ghetto, blade roast, blighted area, bordello,
   breast, brisket, brothel, business district, cathouse,
   central city, chuck, chuck roast, city center, clod, cold cuts,
   core, crib, den, den of vice, disorderly house, dive, downtown,
   filet mignon, flank, ghetto, greenbelt, house of assignation,
   house of joy, house of prostitution, inner city, joint, knuckle,
   loin, midtown, outskirts, panel den, panel house, plate,
   plate piece, pot roast, rack, red-light district,
   residential district, rib roast, ribs, roast, rolled roast, round,
   rump, rump roast, run-down neighborhood, saddle, shank,
   shopping center, short ribs, shoulder, shoulder clod,
   sink of iniquity, sirloin, skid road, skid row, slum, slums,
   sporting house, stew, stews, suburbia, suburbs, tenement district,
   uptown, urban blight, whorehouse