Etymology

• Popular etymology references the 1896 edition of the International Cloud Atlas which defined ten types of cloud. The book defined the ninth cloud as the cumulonimbus, which rises to 10 km (6.2 miles), the highest a cloud can be. No conclusive evidence, however, confirms this origin.
• The 1960 print of the Dictionary of American Slang (Wentworth & Flexner) includes “cloud seven”, 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, no evidence prefers these versions.

In traditional Chinese Han mythology there are nine layers of the sky. The ninth cloud 九霄 is the highest part of the sky or, in Taoism, heaven where the gods live. It also means the palace where the king lives or the king himself. Nine in traditional Chinese culture means infinity. Reference: http://baike.baidu.com/subview/347006/19232896.htm