Refers to a place where criminals, outlaws, or unsavory people gather; carries a negative, secretive connotation. Often used in compounds like 悪の巣窟 (den of evil).
That area is known as a den of criminals.
The police raided the smugglers' lair.
Compound of 巣 (su, 'nest') and 窟 (kutsu, 'cavern' or 'den'). The combination conveys the idea of a hidden, enclosed space where undesirable elements gather.