At the very pinnacle of bliss; euphoric; on a cloud: She came back home and he's on cloud nine(1950s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with on cloud nine
Blissfully happy, as in Ever since he proposed to her, her parents have been on cloud nine. The exact allusion of nine in this term is unclear, and different figures, especially seven (perhaps alluding to seventh heaven), are sometimes substituted.
[ ; mid-1900s