The whole nine yards

Meaning
All of it - full measure.

Origin
Of all the feedback that The Phrase Finder site gets this is the phrase that is asked about the most often. At the outset it should be said that no one is 100% sure of the origin, although many have a fervent belief that they do. These convictions are unfailingly based on no more evidence than 'someone told me'.

Abstract
This piece is quite long, so here's a summary:

Many people are convinced they know the origin of this expression, which has numerous speculative derivations, but aren't able to provide any documentary evidence whatsoever to support their belief of choice.
The earliest known citation of the phrase in print is from 1964, which argues strongly against any of the supposed mediaeval, Victorian or even World War II origins.
The weight of circumstantial evidence, based on the number of early citations, is that the phrase originated in the American military, although the context of the coinage is uncertain.

How was the phrase derived?
"The whole nine yards" crops up in many contexts, which isn't surprising, as there are many things that can be measured in linear, square or cubic yards - and there are also yard-arms, steelyards etc. to account for. This is the source of the variability of the many plausible, but of course mostly incorrect, explanations of the phrase's origin. Regrettably, plausibility doesn't get us very far, as the following will show. The early citations of the phrase don't in fact refer to yards of any particular material, just to a non-specific measure - 'yards'.

The most probable source of the phrase is the US military - that's where many early references to the phrase originate.

The earliest such military reference is from the 1960s, in Elaine Shepard's novel about the Vietnam War - The Doom Pussy (A narrative about the Vietnam War and the men who are fighting it). The book was first published in 1967 and recounts army life during the early 1960s.

The whole nine yards is used several times in the book, principally by the character Major 'Smash' Crandell. The first citation relates to his extracting himself from an unwanted marriage:

The story began when he had absent-mindedly gone through a wedding ceremony a couple of years before while snockered one Saturday night in San Francisco. Slipping out of the knot was expensive but Smash was eventually able to untangle what he called "the whole nine yards."

A later reference concerns a letter to a serviceman from a sweetheart, promising him comprehensive sexual favours when he gets back home. His response to this is:

God. The first thing in the early pearly morning and the last thing at night. Beds all over the gahdam house. The whole nine yards.

It is possible that the phrase was coined by servicemen in Vietnam. One possible source for this would be the Montagnard hill tribes, who were known by the US forces as 'the Yards'. In 1970, the US author Robert L. Mole published The Montagnards of South Vietnam: A Study of Nine Tribes. Some reports suggest that these nine tribes are the source of the 'nine' in TWNY; other US service memoirs claim that Special Operations Group teams consisted of three US soldiers and nine Yards. The disparity in these reports gives some cause for caution, but it could be that the phrase did originate in Vietnam and that Elaine Shepard picked it up as force's jargon while researching for her book.

The military are also the source of the majority of hearsay accounts of the phrase's source. Many of these are of the 'I was there' variety and carry more authority than the usual, and frankly unhelpful, 'I was told' stories. Having spent some time researching this phrase I have received many such reports from servicemen (usually U.S. servicemen). One such example is from a U.S. drill sergeant who claims that the phrase originated in Fort Benning, Georgia, where soldiers were trained in the 'tree-second rush'. This involved running nine yards in three seconds before diving to ground to avoid sniper fire. Of all the explanations I've heard this one seems to me to be the most believable and certainly fits the phrase's meaning, although without documentary evidence it is just another plausible story.

When was it coined?
Although the precise derivation of a given slang phrase is often difficult to determine, the date of its coinage usually isn't. Phrases that are accepted into common use appear in newspapers, court reports, novels etc. very soon after they are coined and continue to do so for as long as the phrase is in use. Anyone who puts forward an explanation of an origin for 'the whole nine yards' which dates it to before the 1960s has to explain the lack of a printed record of it prior to 1964. If, to take the most commonly repeated version for instance, the phrase comes from the length of WWII machine gun belts, why is there no printed account of that in the thousands of books written about the war and the countless millions of newspaper editions published throughout the 1950s and 60s? The idea that it pre-dates the war and goes back to the 19th century or even the Middle Ages is even less plausible.

What I am sure of is that the phrase wasn't in wide circulation before 1961 - which tends to rule out many of the suggested sources. Why? In May 1961, the American athlete Ralph Boston broke the world long jump record with a jump of 27 feet 1/2 inch. No one had previously jumped 27 feet. This was big news at the time and widely reported. Surely the feat cried out for this headline?:

"Boston goes the whole nine yards"

And yet, not a single journalist worldwide came up with that line, which is missing from all newspaper archives. The phrase may have been coined before 1961, but it certainly wasn't then known to that most slang-aware of groups - newspaper journalists.

The likelihood that the phrase originated in the mid 20th century is supported by the lack of any evidence prior to the early 1960s and the ample printed citations from the late 1960s. "The whole nine yards" was in wide enough circulation in the USA then for it to be appearing in newspaper adverts. There are many examples of this, as here from the Playground Daily News, Fort Walton Beach, Florida, 1st May 1969:

'Four bedroom home, located in Country Club Estates. Running distance from Golf Course. Completed and ready to move in. This home has "the whole nine yards" in convenience.'

Earliest citations in print
The earliest known example of the phrase in print that I know of is in the US newspaper The Democratic Standard, 14th March 1855. The story it appeared in was a work of fiction rather than of news reporting and was reproduced in several US papers in 1855. It concerned a judge who arrived at an event without a spare shirt and decided to have one made for him. As a joke a friend ordered one with three times the required material, i.e. 'nine yards of bleached domestic and three yards of linen'. The outcome was:

"He found himself shrouded in a shirt five yards long and four yards broad. What a silly, stupid woman! I told her to get enough to make three shirts; instead of making three, she has put the whole nine yards into one shirt!"

Well, that does contain the phrase in question and it does relate to yards of material, which is one of the commonly repeated origins. This appears to be by pure chance though. After all, the individual words are common enough and have to appear together arbitrarily sometimes. This can't be accepted as the origin.

To get a more plausible source we have to come forward to as recent a date as 1964, which is the earliest date I've yet found for the 'whole thing' meaning of the expression. On 18th of April that year, the Texas newspaper The San Antonio Express and News reprinted an article headed How To Talk 'Rocket', by Stephen Trumbull. He wrote the piece for The World Book Encyclopedia Science Service and it lists and explains new jargon terms that were in use in the space exploration community in the USA. He offered the opinion that "the new language spreads across the country - like a good joke - with amazing rapidity", which suggests that the terms listed were recently coined, and he went on the write:

"Give 'em the whole nine yards" means an item-by-item report on any project.

Whether the term actually originated as spaceman's jargon is open to doubt. It could easily have been appropriated by them from another source. That source could well have been the US military, as that's where many early references to the phrase originate.

Suggested derivations
Despite being sure they are all inventions, I'm obliged to include some of the versions of the source of the phrase that are going the rounds. Take your pick, and feel free to make up your own, everyone else does:

It comes from the nine cubic yards capacity of US concrete trucks and dates from around 1970s. Widely circulated although arrant nonsense as even the largest concrete mixers were smaller than 9 cubic yards in 1967.

The explanation refers to World War II aircraft, which if proved correct would clearly predate the concrete truck version. There are several aircraft related sources:

The length of US bombers bomb racks.
The length of RAF Spitfire's machine gun bullet belts.
The length of ammunition belts in ground based anti-aircraft turrets, etc. No evidence to show that any of these measured nine yards has been forthcoming.

Tailors use nine yards of material for top quality suits. Related to 'dressed to the nines'?

The derivation has even been suggested as being naval and that the yards are shipyards rather than measures of area or volume. Another naval version is that the yards are yardarms. Large sailing ships had three masts, each with three yardarms. The theory goes that ships in battle can continue changing direction as new sails are unfurled. Only when the last sail, on the ninth yardarm, is used do the enemy know which direction the ship is finally headed.

A mediaeval test requiring the victim to walk nine paces over hot coals.

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Source: http://www.phrases.org.uk/meanings/the-whole-nine-yards.html