Here's the breakdown:
* Jupiter was the king of the gods in Roman mythology, associated with the sky, thunder, and law.
* The Romans associated him with joy and festivity because he was often depicted as a jovial and generous god.
* The Latin word for Jupiter was "Iuppiter", and a derivative of this, "Iovis", was used to refer to Jupiter as a personification of joy and festivity.
* "Jovial" comes from the Latin adjective "jovialis", which means "pertaining to Jupiter" or "of Jupiter".
Therefore, the word "jovial" has its origins in the ancient Roman belief that Jupiter was a joyful and generous god. Over time, the word came to be used to describe anyone who was cheerful, optimistic, and full of good humor.