Definition & Meaning of "on cloud nine"

What is the origin of the idiom "on cloud nine" and when to use it?

The exact origin of the phrase "on cloud nine" is uncertain, but it is believed to have emerged in the mid-20th century. It is commonly used in romantic contexts to describe the elation of being in love or experiencing a deep emotional connection. It can also be used to express sheer happiness and contentment in general.