Word Origin & History
by 1950, sometimes also cloud seven (1956, perhaps by confusion with seventh heaven), Amer.Eng., of uncertain origin or significance. Some connect the phrase with the 1896 International Cloud-Atlas, long the basic source for cloud shapes, in which, of the ten cloud types, cloud No. 9, cumulonimbus, was
the biggest, puffiest, most comfortable-looking. The phrase might appear in the aviation-based play "Ceiling Zero" by Frank Wilber Wead (1935).