Whole nine yards
Comes from World War II fighter pilots, whose planes typically were
outfitted with enough machine-gun ammunition to extend 27 feet. If a pilot
expended all his ammunition on a mission, he would say, "I gave them the
whole nine yards".

A site visitor also wrote in with his/her explaination of this idiom:
You might want to check out the older origin of the term" whole nine yards", the original term comes from the Scottish Great Kilt. The amount of wool for a full Great Kilt is a "whole nine yards".

another site visitor also wrote in with this:
Whole nine yards refers to the total amount of concrete held by a concrete truck. Concrete is measured in the unit 'yards'. The truck holds a total of nine yards of concrete. When all the concrete was needed they would say, give me the Whole nine yards.

(来源：英语交友 http://friends.englishcn.com)