The idiom whole nine yards speaks to the extent of someone’s effort. If you go the whole nine yards, you thought of everything, you used every resource, you looked into every possibility. Or if it is used as mass noun to describe a group, that group includes everything possible. See the examples below for more clarity on this idiom’s use.
The idiom originated in the United States some time in the first half of the twentieth century. There is no evidence, however, to know what nine years the phrase is referring to. A popular theory is that it takes nine yards to make a Scottish kilt. However, while the length fits, there is little to say that it would turn into a phrase which means to its fullest extent. Other theories about cement mixers and ammunition belts are false.