The origin of sense 1 (“a state of bliss”) is uncertain; however, the following etymology has been suggested:
The first edition of the International Cloud Atlas (1896),[1] which defined ten types of cloud, described the ninth type as the cumulonimbus which rises to 10 km (6.2 miles), the highest a cloud can be.[2]
Compare cloud seven(“state of complete happiness or euphoria”),[3] which may have originated from confusion of cloud nine with seventh heaven.[2]
Sense 2 (“a state of fantastic or impractical dreaming or thinking”) may be due to a confusion between sense 1 and the phrase head in the clouds.
He was on cloud nine for days after she agreed to marry him.
1978, Watchman Nee, “Living by Faith”, in The Salvation of the Soul: Translated from the Chinese, New York, N.Y.: Christian Fellowship Publishers, →ISBN, page 107:
In your zeal you may not feel you are undertaking a thing for your own sake but think you are doing so for God. Nevertheless, you should remember that at the time of great excitement as though you were on cloud nine, you may perhaps be most fleshly!
Following a highly successful tour of Italy in July/August 1993, with recital and orchestral appearances and an especially inspiring last performance, I returned to New York on cloud nine and full of euphoria.
[2000 May, Eknath Easwaran, chapter 12, in A More Ardent Fire: From Everyday Love to Love of God, Tomales, Calif.: Nilgiri Press, →ISBN, page 142:
Most of us do not think of excitement as a problem. After all, doesn't everybody like to be on cloud number nine? I agree: if we could stay on cloud number nine, life might be very pleasant. But, as all of us know, the cloud eventually evaporates. Then we not only come abruptly back to earth; we usually burrow right into its depths and hide – that is, we go into a depression.]
2008, Melissa de la Cruz, “Not Quite Ready for Her Close-up”, in Lip Gloss Jungle (Ashleys; 4), New York, N.Y.:Aladdin Mix, →ISBN; republished as Popularity Takeover (The Ashley Project), New York, N.Y.: Aladdin Mix, 2015 April, →ISBN, pages 176–177:
A. A. scanned the crowd and spotted Lili's happy face. Lili had been on cloud nine ever since she and Max had gotten back together.
1970 July 16, W. E. Cushen, “The Management of Change”, in Frances C. Bell, Harold F. Wollin, editors, Report of the 55th National Conference on Weights and Measures 1970: Sponsored by the National Bureau of Standards; Attended by Officials from the Various States, Counties, and Cities, and Representatives from U.S. Government, Industry, and Consumer Organizations, Salt Lake City, Utah, July 13–17, 1970 (NBS Special Publication;342), Washington, D.C.:National Bureau of Standards, United States Department of Commerce, published April 1971, →OCLC, page 152:
Now, the system approach, needless to say, wants to help out in that particular problem, but it lives up on cloud nine for the most part. This is one of the reasons that it has not made more headway than it has in the civilian agencies of government.
1973 May, Robert Schindler, “Puget Sound Region—Establishing a Continuing Program [Question Period]”, in Jacob Silver, editor, Geographic Base File System—Establishing a Continuing Program: Conference Proceedings: January 18 and 19, 1973, Seattle, Wash. (Computerized Geographic Coding Series;GE60 no. 4), Washington, D.C.:Bureau of the Census, Social and Economic Statistics Administration, U.S. Department of Commerce, →OCLC, page 26, column 2:
Mr. Walsh – […] It sounds like the purpose of the geographic base coding system is to fulfill a planning function of the agency. If that is not the case, then I am concerned as that is one way to get up on "cloud nine"—that is, to not associate with end products.
1981 July 28, Basil Arthur, “Appropriation Bill—Financial Statement”, in Parliamentary Debates (Hansard): Third Session, Thirty-ninth Parliament:House of Representatives, volume 439 (Comprising the Period from 17 July to 7 August 1981), Wellington, N.Z.: P. D. Hasselberg, government printer, →OCLC, page 2049:
Members are living on cloud nine if they think that playing the stock-numbers game is doing anything for the farmers or the country.
By coming to grips with these problems, we can demonstrate that libertarianism is not just a beautiful ideal somewhere on Cloud Nine, but a tough-minded body of truths that enables us to take our stand and to cope with the whole host of issues of our day.