A state of bliss, elation or happiness.
Whenever a phrase includes a number, like the whole nine yards, at sixes and sevens etc., then attempts to find its derivation usually focus on the number. 'On cloud nine' is no exception. A commonly heard explanation is that the expression originated as one of the classifications of cloud which were defined by the US Weather Bureau in the 1950s, in which 'Cloud Nine' denotes the fluffy cumulonimbus type that are considered so attractive. Another explanation is that the phrase derives from Buddhism and that Cloud Nine is one of the stages of the progress to enlightenment of a Bodhisattva (one destined to become a Buddha). (5 years and 643 days ago)