cloud nine (n.)

by 1950, sometimes also cloud seven (1956, perhaps by confusion with seventh heaven), American English, of uncertain origin or significance. Some connect the phrase with the 1895 International Cloud-Atlas (Hildebrandsson, Riggenbach and Teisserenc de Bort), 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. Shipley suggests the sense in this and other expressions might be because, "As the largest one-figure integer, nine is sometimes used for emphasis." The phrase might appear in the 1935 aviation-based play "Ceiling Zero" by Frank Wilbur Wead.