During the Vietnam war, some Montagnard tribal leaders would fail to list casualties so that the pay and food allowances to them would continue coming in by their Special Forces and CIA benefactors. Too, sometimes even the wounded and disabled, missing an arm or leg, would try to get in on the payroll.
So prior to raids and patrols, as well as payday, headcounts and inspections of each of the squad-sized teams of tribal guerrilas had to be made to ensure that all the participants were both present and in sound condition: the whole nine 'Yards....
Anyway, that's my story, and I'm stickin' to it....
-archy-/-
A three mast sailing ship carried nine yards three per mast - hence, the whole nine yards.