Historically, "turnkey" refers to a person in charge of the keys, particularly at a prison. The word derives from the combination of "turn" and "key," a literal description of the jailer's actions.
"Turnkey" or "turn-key" can also refer to an arrangement under which a single contractor is entirely responsible for the construction of a house or building, which is then sold when it is completely ready for operation or occupancy. In this sense, the project is not complete until the key is ready to be turned by the owner. "Turnkey" can also refer to any operation that is fully equipped and ready to open, as in a turnkey business.