Like Johanna, I knew the “bad cess” expression. I did know it was Irish, but until today I always (wrongly) thought it was pronounced kess, from Irish ceas, “oppression; sorrow; debility.” So I never connected it with cesspool.
I gather that this is a good thing, because there is no connection between them (in the same way that there is no reason to connect, for instance, the words “fun” and “funeral”).
As for the etymology, I rather liked the Random House one:
Interesting that even on the site that offers that, there appears to be a case of dueling etymologies But the reason I like that one is because someone who speaks Italian could conceivably tell us whether cesso is really an Italian word for “privy.”
Oh, and that it gives a date range for first usage.
All of these seem to go back to the Latin root cess-, which literally means ‘to go’.
abscess - go away
access - go toward
ancestor - gone earlier
antecedent - going earlier
cession - going
concession - going with
decease - go down
excess - go out
incessant - not going away
intercession - going between
precession - going before
process - go forward
recess - go back
retrocession - going backwards
secession - going by oneself
success - go under
Cool. All y’all.
ETA: and to think, I was just riffing on cesspoll/cesspool.