What is the 9 in cloud 9 mean?

Answer 1

The use of the word "nine" to denote greatness or "the best" goes back to medieval times. The Nine Worthies, usually called simply The Nine, were well-known to medieval scholars as the personification of all that was noble and heroic. The Poetick Miscellenies of Mr John Rawlett, 1687, provides the earliest reference to 'to the Nine':

The learned tribe whose works the World do bless,

Finish those works in some recess;

Both the Philosopher and Divine,

And Poets most who still make their address

In private to the Nine.

"Nine" has been used in numerous phrases, such as "dressed to the nines," and "cloud nine" to denote that this is the ultimate or best that it gets.