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Having strong feelings of happiness or satisfaction.
It's thought that this phrase may have come from meteorologists, who sometimes classify different types of clouds by using numbers, depending on the cloud's altitude. Clouds labelled with the number 9, for example, were considered high in altitude. Thus, a person who is feeling quite happy sometimes describe themselves as floating high up on the clouds, or more specifically, floating on cloud nine, since these are considered to be the highest.

The expression is used in the Denton Record Chronicle, May 1949, when a woman named Betty Hutton was thrilled to have been accepted for a movie role:

"Let's look in on Betty Hutton, who says she is hovering 'on Cloud No Nine' these days."
* Dallas feels as though he's floating on top cloud nine, because he finally found a job after months of earnest searching.
Note: The origins for most popular idioms cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so.

In addition, quotes that contain a phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means shows they originated from these. In all likelihood, if an expression is being used in a newspaper, it's probably already well known, and thus, from an older period of time.
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