cesspool:  Cesspool has no direct etymological connection with pool. It comes from Old French suspirail ‘ventilator, breathing hole’, a derivative of souspirer ‘breathe’ (this goes back to Latin suspīrāre, source of the archaic English suspire ‘sigh’). This was borrowed into English in the early 15th century as suspiral ‘drainpipe’, which in the subsequent two hundred years appeared in a variety of spellings, including cesperalle.
By the early 16th century we find evidence of its being used not just for a pipe to drain matter away, but also for a well or tank to receive matter thus drained (originally any effluent, not just sewage). The way was thus open for a ‘reinterpretation’ of the word’s final element as pool (by the process known as folk etymology), and in the late 17th century the form cesspool emerged.
By analogy, as if there were really a word cess ‘sewage’, the term cesspit was coined in the mid-19th century. => suspire
also cess-pool, 1670s, the first element perhaps an alteration of cistern, 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," 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."