cesspool (n.)

also cess-pool, "cistern or well to receive sediment or filth," 1670s, the first element perhaps an alteration of cistern, or perhaps a shortened form of recess [Klein]; or the whole may be an alteration of suspiral (c. 1400), "drainpipe," from Old French sospiral "a vent, air hole," from sospirer "breathe," from Latin suspirare "breathe deep" [Barnhart]. 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" [OED], 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."