Cloud 9 is an idiom for a state of joy, euphoria, or extreme happiness. Usually "cloud 9" is coupled with the "on" preposition.

"Those chocolate bunny ears were so good I was on cloud 9! I love consuming simulated fluffy animals!"

There are a couple theories regarding this expression's origin. An early cloud classification system had nine levels of cloud classification. The cumulonimbus clouds, the most fluffy cotton candyish of clouds, were slotted in at the 9 position. Clouds, heaven, angels have long been associated. A true state of euphoria might be considered being up on the cloud 9 level, upon the most aesthetically pleasing of clouds.

Alternatively, the number 9 is considered a very pure number. It's a trinity of trinities. Three times three. Hence the expression "dressed to the nines", which means well dressed. So being up on a cloud numbered 9 is surely a good thing, if you adhere to a childhood fantasy of heaven. For those who believe oxygen deprivation has the greater downside, well, it's a somewhat silly allusion.

A strong possibility for the origin comes to us from Buddhism. In Buddhism one ascends 10 clouds. Cloud 9 is a state of enlightenment, where one's acts are done without regard for the self. However, the expression's popular use makes this suspect. Being on cloud 9 almost always refers to a rather selfish state of self satisfaction.