The Collaborative International Dictionary of English v.0.48:

Cesspool \Cess"pool`\ (-p[=oo]l`), n. [See Sesspol.]
   A cistern in the course, or the termination, of a drain, to
   collect sedimentary or superfluous matter; a privy vault; any
   receptacle of filth. [Written also sesspool.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

cesspool
    n 1: a covered cistern; waste water and sewage flow into it
         [syn: cesspool, cesspit, sink, sump]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "cesspool":
   Sodom, bog, cesspit, cloaca, cloaca maxima, den, drain, dump,
   garbage dump, marsh, mire, pandemonium, quagmire, septic tank,
   sewer, sink, sty, sump, swamp