Where does the expression the whole nine yards come from?
We're stumped. There appears to be no clear or simple answer to your question.
None of the dozen or so dictionaries we checked contain a straightforward answer on the origin of the whole nine yards. One online source, World Wide Words, calls this phrase "one of the great unsolved mysteries of modern etymology." The only thing that authorities seem to know for certain is that it dates from the 1960s and is American in origin.
The Internet yields many theories on the origins of this phrase. Here is a list of what the whole nine yards could refer to:
Most of these theories are very doubtful, so your guess is as good as ours!