WordNet (r) 3.0 (2006):

cathouse
    n 1: a building where prostitutes are available [syn:
         whorehouse, brothel, bordello, bagnio, house of
         prostitution, house of ill repute, bawdyhouse,
         cathouse, sporting house]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "cathouse":
   bagnio, bawdy house, bawdyhouse, bordello, brothel, clip joint,
   crib, den, den of thieves, den of vice, disorderly house, dive,
   dump, fleshpots, gyp joint, hole, house of assignation,
   house of joy, house of prostitution, joint, lair, panel den,
   panel house, red-light district, sink of iniquity, sporting house,
   stew, stews, tenderloin, whorehouse