I do not think anyone really knows for sure...
These are some of the versions going the rounds: without evidence one is as good as another, take your pick...
It comes from the nine cubic yards capacity of US concrete trucks and dates from around 1970s. I know from personal experience of working on the Spaghetti Junction construction site in the UK that trucks often 'lost' part of their load between the mixing depot and the proper destination. The many concrete forecourts in the West Midlands area bear continuing witness to that. A full load delivered to the roadworks was a rarity and was usually commented on, so a phrase being coined to mark the event seems believable - not in the UK though as concrete trucks here at the time carried seven cubic yards.
The explanation refers to World War II aircraft, which if proved correct would clearly pre-date 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.
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.
"If you're not grateful to live in America, you're not paying attention."
