Being on cloud nine is a metaphor for a feeling of euphoria and unmitigated bliss. Cloud nine is also the meteorological term for cumulonimbus clouds. The phrase originated in the 1950s, according to Merriam-Webster.
Origin of the phrase is often attributed to the field of meteorology and the numbering system used to classify clouds, in particular. However, similar numbered cloud references supposedly precede any documented use of "cloud nine." There is evidence of references to a sublime state as "cloud seven," for instance. Another term combining numerology with a celestial reference, "seventh heaven," serves as a synonym for cloud nine.