noun
underworld; underside of society
Refers to the hidden, often criminal side of society, including organized crime, black markets, and other illicit activities.
彼は裏社会とのつながりを疑われている。
He is suspected of having connections with the underworld.
裏社会の人間は、表の顔と裏の顔を使い分ける。
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'.