These are some of the versions going the rounds: take your pick...
- It comes from the nine cubic yards capacity of US concrete trucks and dates from around 1970s.
- 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, 1. the length of US bombers bomb racks, 2. the length of RAF Spitfire's machine gun bullet belts, 3. 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.
If anyone has any hard evidence of this phrase being used before 1967, e.g. an appearance of the phrase in print, we would love to see it. Please post your feedback at the Phrase Finder Discussion Forum - but please, evidence not conjecture.