noun
1. : a room situated in the rear ; especially : one accessible only to special or private groups
in the back room of every tavern, clubs held meetings several times a week — Bernard Faÿ
2. : the meeting place of a leadership or directing group (as of a political party) that wields its influence and authority in an informal, inconspicuous, and often indirect way reaching decisions through negotiation among its members and sometimes making use of dubious or underhanded methods
the man in the back room, avoiding direct contacts, weaving a web of intrigue and influence — M.W.Straight
delegates … were merely marking time until the real decisions were made in the back rooms — Time