cesspool

<; or the whole m. Meaning extended to "tank at the end of the pipe," which would account for a possible folk-etymology change in final syllable.

Other possible etymologies: Italian cesso “privy,” from Latin secessus “place of retirement” (in Late Latin “privy, drain”); dialectal suspool, from suss, soss “puddle;” or cess “a bog on the banks of a tidal river.”