the.com/back room

means A private space (literal or metaphorical) where decisions, deals, or arrangements are really hashed out, away from public meetings and formal process.

from Literal at first — the back room of a shop, pub, or hall, physically behind the public-facing front room. Because such rooms were private and out of sight, they became the natural setting for the deals, haggling, and political horse-trading people didn't want done in the open. By the 1800s-1900s 'back room' and 'back-room boys/deals' had shifted into figurative use, especially in politics and business, to mean any process of real decision-making happening behind the scenes of the official, public one.