Wordorigins.org

The phrase on cloud nine describes a state of euphoria or elation and the reference is of unknown significance. It is an Americanism and is first recorded in 1957 in Elliot’s Among the Dangs:

I waited awhile, but he was off on cloud nine.

Although the phrase with the number nine dates to 1957, there are variants using other numbers dating back to the 1930s. Albin Jay Pollock’s The Underworld Speaks from 1935 has:

Cloud eight, befuddled on account of drinking too much liquor.