Red-light district - Idioms by The Free Dictionary
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An area in a city containing brothels and other businesses related to activities that are typically illegal elsewhere. If you patronize the red-light district, you're essentially subsidizing the sex trade.
red-light district an area of a city or town containing many brothels, strip clubs, and other sex businesses.
the red-ˈlight district the part of a city where prostitutes work and sex shops are found: I got lost and found myself in the red-light district all alone!