As a noun, turnkey is one word and means the person who is in charge of the keys of a prison. In this form the word has been in existence since the mid-1600s. The plural is turnkeys.
Turnkey can also be an adjective describing an object as completely finished and able to be used. This form was coined in the 20s and is usually used in marketing. It can also be used to describe tasks or jobs that are easily done or require little effort.
Sometimes the word is seen hyphenated, but this is an error, probably from those who think the adjective is a compound of the words turn key.
Pop’s mission is to change the world of extras casting by providing any company across the globe wishing to book extras, a turnkey solution to do so: a book of artists, time-saving software to cast and co-ordinate them, and the complete back office service to arrange their payment. [The Guardian]
That foreign influx is pushing developers to create ever more elaborate island enclaves, built to satisfy the turnkey lifestyle demands of the world’s upper class. [New York Post]
Business executives still see CIOs as IT turnkeys, not stewards of strategic technologies that can help acquire customers or grow revenue, according to a new Forrester Research Inc. survey. [The Wall Street Journal]
At the same time, he stated while bad turnkeys should be disciplined, instances of prisons officers abusing inmates are in the minority. [Newsday]