The popular etymology references the 1896 edition of the International Cloud Atlas which defined ten types of cloud. The ninth cloud was the cumulo-nimbus rising to a height of 10 km, which is the highest a cloud can be. There is, however, no evidence for this.

The 1960 print of the Dictionary of American Slang (Wentworth & Flexner) includes “cloud seven”, the usage of which seems to predate “cloud nine” by a few years. Other etymologies reference Buddhist or Christian lore, and even Dante’s Divine Comedy, but again there are no reasons to prefer these versions.