Wiktionary
n. (context idiomatic English) A place of immoral behavior, usually of a sexual type.
Usage examples of "den of iniquity".
Time to pack it in before she keeled over and spent the day passed out in this den of iniquity.
Maybe old Vandoise was blowing up the cave because he thought it was a den of iniquity.
At once, the Pavilion of Cynosure became an abhorrent den of iniquity, strewn with couches and pillows and filled with such a fog of sweet-smelling perfumes and bitter smoke I could hardly breathe.
It's yet another appalling den of iniquity, and at the sight of the head of the Sorcerers Guild striding through the doors, the place goes quiet.
Given the foul nature of his sister, he should have known from the very beginning that the whore would fly to such a den of iniquity at the first opportunity, like a moth to a flame.
There were more ordinary businesses here: cafes, tanneries, and dye shops, but from a nighttime view the place looked like nothing but a den of iniquity.