What is the origin of the term "screw" in the case of a prison guard?

The term screw can refer to a prison guard. An example of this is seen in the folk song The Catalpa:

So come all you screw warders and jailers
Remember Perth regatta day
Take care of the rest of your Fenians
Or the Yankees will steal them away

My question is, where did this term come from? I have found this page which provides an explanation, but there are no citations as to where it got it's information from. There is also this page which provides two different explanations, both of which have sources.

Which is correct and are there sources to back it up?

Solution 1:

In complement to Kosmonaut's answer, I'd like to add a few pieces to the jigsaw puzzle.

The undisputed etymology of the English noun screw is from Middle French "escroe" (pronounced "escrow") which evolved into present-day French "écrou" (pronounced a-crew) and designates the nut (of a bolt). Its use in English is recorded as early as ca 1400.

Interestingly enough there are in present-day French a number of expressions related to the jail system bearing the word "écrou".

• écrouer: to imprison.
• registre d'écrou: the register log where new incarcerations and releases are recorded along with the cause of imprisonment.
• numéro d'écrou: the unique id for a prisoner in a given jail.
• levée d'écrou: the release of a prisoner (literally raising the screw).

From there one is faced with two different and possibly complementary explanations because the Old French word escroe has two different meanings, each with its own etymology.

1. The first (ca 1160) meaning of the Old French word escroe is that of a scroll to which new strips (called escroeles) of parchment where appended when more room was needed. From this meaning comes the posterior English words scroll and escrow. This meaning in turn evolved to also designate various royal administration registers (for instance "écroues des dépenses du Roy"). Another of these registers was used to keep track of the imprisonments and releases of prisoners. Hence the "registre d'écrou" and the word "écrouer".

2. Oddly enough the second meaning (16th century) of the Old French word escroe is that of the common screw. Although the etymology is still disputed, the most convincing theory is that of an analogy with the genitals of the swine and the boar (the penis of a boar is shaped like a cork-screw and the swine cervix matches that shape). The Latin word for a breeding swine is scrofa ^{1, 2}.

So how does the screw relate to a key?

First one has to take into account the fact that many prisoners were not only locked in cells (either individual or collective) but also shackled and chained to the wall (in older times when locks were expensive to produce, they were just chained) and that involved shackle riveting and later screwing (for screw pin shackles). There are a number of collectors shackles that can illustrate this "technology" - here is a randomly selected sample (of which I include the pictures below in case it goes away). One can guess how it works: the screw must first be removed so that the key can open the shackles.

Solution 2:

The OED first cites screw in the 19th century.

Etymonline (usually a pretty good source) gives this explanation:

Meaning "prison guard, warden" is 1812 in underworld slang, originally in reference to the key they carried.

Looking up the key connection I found this source:

screw as a term for a prison guard is based on the fact that screw was originally slang for "key." One of the most important functions of a prison guard, or turnkey, as he's often called, is to see that prisoners are locked up at the appropriate times -- and that involves turning the "screw." Interestingly enough, Henry Mencken reports in The American Language that in the 1920s deskmen and bellboys in hotels used screw as a slang term for room key. Another theory is that screw refers to the thumbscrews used by jailers in ancient times to torture prisoners into confessing.

Assuming the content of this webpage isn't totally fabricated, this info came from the Encyclopedia of Word and Phrase Origins.

So, the origin is not for certain; it seems likely that it had something to do with the key they carried, but might possibly had some connection to thumbscrews.

Solution 3:

This came from the prison's punishment pointless device the crank. This was a large handle in a prisoner's cell that they would have to turn, thousands of times a day. This could be tightened by the warders, making it harder to turn, which resulted in their nickname of screws. These punishments were not abolished until 1898.

Solution 4:

A prison guard is sometimes referred to as a "turnkey." Therefore "screw" is a slang version of this word.