underworld; underside of society
Refers to the hidden, often criminal side of society, including organized crime, black markets, and other illicit activities.
He is suspected of having connections with the underworld.
People in the underworld maintain a public face and a hidden face.
Refers more specifically to a physical area or district associated with crime and vice, like a 'sinister quarter'.
Compound of 裏 (ura, 'back, reverse side') and 社会 (shakai, 'society'), literally meaning 'the back side of society'.