The phrase "on cloud nine" is used to mean elation and joy or a giddy sense of happiness. It describes a person's emotional state in a positive way, usually in response to an exciting or pleasing event that has occurred.
The etymology of the phrase is not entirely clear, with several different suggestions for the phrase "on cloud nine" but no conclusive proof of origin. The International Cloud Atlas of 1896 defined 10 types of clouds, with number nine being the cumulonimbus, which travels the highest and is the fluffiest, most comfortable-looking cloud. This is considered the most popular etymological reference, though other texts reference "cloud seven" as a precursor to the popular phrase.