I'll go over some of the more common suggestions, but first let's consider some important background material. The phrase is first found, to my knowledge, in 1966. (An unreliable book has claimed that it dates from the 1950s, which is itself not that implausible.) The early examples do not seem to be associated with a particular field; for example, it is found in military sources, but it doesn't seem to be a specifically military expression. The phrase is an Americanism.The alt.usage.english FAQ has the following for whole nine yards:
A reasonable etymological theory must meet several criteria. It must be internally true--you cannot claim that the whole nine yards comes from the fact that a man's three-piece suit require nine square yards of cloth if such a suit only requires five square yards. It must jibe with the evidence we have--an origin in some colonial practice is not likely to be the origin of a term first found in the 1960s. It must be sociolinguistically plausible--an origin in the jargon of cement-truck operators is unlikely because there's no reasonable way that cement- truck-operator jargon would make it to general use. There are other criteria, but these are rough guidelines of what we can demand from an explanation. With that in mind, let's look at what some people have said.
Despite the use of yards as the standard measurement of distance in football, nine yards is not a significant distance in the game. No one has ever discovered a quote from, say, a movie about football or from a famous football player about "going the whole nine yards" in reference to a particularly important play.
It is asserted that a standard capacity of cement-mixers is nine cubic yards, and that a full load would be "the whole nine yards." There is no standard capacity for cement mixers--current models vary between seven and ten cubic yards--but in the 1960s, when the phrase was first used, they carried about four cubic yards of cement, and six cubic yards was considered extremely large. Also, it's unlikely that a phrase from cement-mixing jargon would make it into the mainstream.
It is asserted that various articles of clothing, such as a man's custom-made three-piece suit, a formal bridal veil or train, or a gown in colonial times, customarily require nine square yards of material, or that material normally comes in bolts of nine square yards. In fact, a man's custom-made suit requires only about four to five square yards of cloth; even the late Princess of Wales' staggeringly long veil was only twenty-five feet (8 1/3 yards) long, and colonial gowns are too old to bother considering. Bolts of cloth are normally twenty or more yards long. Finally, the garment industry is again not a likely source of slang.
It is asserted that the "yard" is not a reference to length, but is rather one of the long spars to which a sail is affixed on a ship; ships had a maximum of nine of these yards, and a ship trying to go as fast as possible with all its sails would be using "the whole nine yards." First of all, ships often had more than nine yards; it depended on the number of masts, but fifteen or eighteen yards were not unusual. Second, seafaring terminology is an unlikely origin for a term only thirty-odd years old. Third, the phrase "all nine yards" would be more likely in this context than "the whole nine yards."
It is asserted that nine yards is a customary length of a burial shroud, and "the whole nine yards" would refer to death, and by extention any extreme, final limit. This suggestion has at least some basis--nine yards is a customary length of burial shrouds in some areas. However, the semantic link doesn't seem likely--it's more of a stretch from "death" to "everything possible" than one would like. The word "whole" again doesn't make much sense in this context. Also, the actual phrase "the whole nine yards" has never been found in conjunction with burial practices.
A more recent assertion is that twenty-seven feet was the standard length of a machine-gun belt, and that firing off the entire round was shooting "the whole nine yards." This is sensible in a number of ways- -the military is often a source for expressions of this type; it makes perfect semantic sense; the phrasing is reasonable. Most machine-gun belts were less than twenty-seven feet, unfortunately, and of course this phrase is not found specifically associated with this theory until very recently.
There are other suggestions, most of which can be dismissed using similar reasoning. Please feel free to send in early (pre-1975) examples if you find them, but before you make an etymological suggestion that your husband's sister's mother's brother swears is true, reflect on the fact that it's probably wrong too.
This phrase, meaning "all of it, everything", dates from at least the 1950s. The origin is a matter for speculation. 9 yards is not a particularly significant distance either in football or in the garment business (a man's three-piece suit requires about 7 square yards of cloth, and cloth is sold in bolts of 20 to 25 yards). The phrase may refer to the capacity of ready-mix concrete trucks, alleged to average about 9 cubic yards. Some people (e.g., James Kilpatrick in _Fine Print: Reflections on the Writing Art_) have satisfied themselves that the concrete-trucks explanation is the correct one; but I haven't seen the evidence. And Matthew Jetmore has unearthed some evidence to the contrary, a passage from the August 1964 issue of _Ready Mixed Concrete_ Magazine: "The trend toward larger truck mixer units is probably one of the strongest and most persistent trends in the industry. Whereas, just a few years ago, the 4 1/2 cubic yard mixer was definitely the standard of the industry, the average nationwide mixer size by 1962 had increased to 6.24 cubic yards, with still no end in sight to the demand for increased payload." The phrase is covered by Cecil Adams in _More of the Straight Dope_, pp. 252-257. A "canonical collection" of explanations has been compiled by "Snopes" (email@example.com).
Michael Nunamaker writes that a friend of his in the U.S. Air Force suggested a World War II origin: "According to him, the length of the ammunition belt (feeding the machine guns) in the Supermarine Spitfire was nine yards. Therefore, when a pilot had shot all his ammunition he would say he had 'shot the whole nine yards'."
This hasn't been updated that recently because one of the more "definitive" definitions is about old sailing ships. A yard was one of the sails. TO have the whole nine yards means being under full fail.
(extract from the alt.quotations archives, article by Daniel P. B. Smith referencing:  "Jesse's Word of the Day" [Jesse Sheidlower] at Random House; and the aue FAQ)
From the "Straight Dope" site: how the origin of the "whole nine yards" may have come from American football
It seems perfectly logical to me that the true meaning of the phrase does indeed spring from [American] football. However, rather than indicating fulfillment of a goal, the phrase probably was originally intended ironically.
In an instance of shortfallen achievement where a disdainful comment would be appropriate, it could be said sarcastically that "he went the whole nine yards." ... --Rick A., Chicago
(extract from the Cecil Adams "Straight Dope" site)
From the "Word Origins" site: a concurring entry
One final possibility is that it does derive from American football, but was originally intended to be ironic. To go "the whole nine yards" was to fall just short of the goal.
(extract from Dave Wilton's Word Origins site
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