The adjective "jovial," from the root "jove," has been used to describe things related to Jupiter, who was the Roman equivalent of the god Zeus of Greek mythology. Both were gods of the sky and were symbolized by the lightning bolt.
Jupiter, also called Jove, was the chief god of the Roman religion until it was replaced by Christianity. In Roman mythology, Jove was responsible for creating the rules of the Roman religion, such as the sacrificial act. Jove's sacred animal was the eagle, which appeared on many Greek and Roman coins, and his sacred tree was the oak. Although the gods were considered to be identical, the renaming of Zeus as Jove by the Romans is interpreted as a movement away from gods that were created to explain natural occurrences, such as thunder and lightning, and toward deities that established social and political norms.
This is the reason Jupiter/Jove also was believed to be the divine witness of oaths. The Romans believed that a good government could only exist upon a foundation of trust and justice. Therefore, Jove also had the responsibility of maintaining the strength of oaths, and in turn justice, upon which the Roman Empire's history-changing political structure was built.