‘The Whole Nine Yards’ Of What?

Where does the phrase “the whole nine yards” come from? In 1982, William Safire called that “one of the great etymological mysteries of our time.”

He thought the phrase originally referred to the capacity of a cement truck in cubic yards. But there are plenty of other theories.

Some people say it dates back to when square-riggers had three masts, each with three yards supporting the sails, so the whole nine yards meant the sails were fully set.