Several origins of the phrase "the whole nine yards" have been suggested, such as the amount of fabric needed to make some particular garment. None of the explanations has any real evidence for it.

from the alt.folklore.urban FAQ:

True. There is no good etymology for the phrase "The whole nine yards."
True. Suggestions have included: Volume in a concrete mixer, coal truck, or a wealthy person's grave; amount of cloth in a man's custom-made (i.e., "bespoke") suit, sports games, funeral shroud, kilt, in a bolt of cloth, square area in a ship's sails, and volume in a soldier's pack.