A pit or tank used to collect and break down sewage and waste water, or figuratively, a place that is dirty, corrupt, or morally bad.
From 'cess' (a tax or levy, later referring to sewage) combined with 'pool.' The term has been used since the 18th century for waste disposal, and gained the figurative meaning of 'corrupt place' by the 20th century.
Before modern sewage systems, cesspools were literally where human waste went to decompose—writers started using it metaphorically for places of moral corruption because of this literal filthiness!
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