The story
From turn + key. (jailer): Compare typologically Russian вертуха́й (vertuxáj) (akin to верте́ть (vertétʹ)).
Etymology from Wiktionary, CC BY-SA 4.0.
turnkey
Today it means: Ready to use without further assembly or test; supplied in a state that is ready to turn on and operate (typically refers to an assembly that is outsourced for manufacture). Full definition →