Jovial \JOH-vee-uhl\ , adjective;
1. Endowed with or characterized by hearty, joyous humor or a spirit of good-fellowship
2. With a capital 'j': Of or pertaining to the god Jove, or Jupiter
Jovial dates to the 1580's from a French word that was taken from Italian joviale (literally "pertaining to Jupiter"). Joviale comes from Latin Jovialis ("of Jupiter") from Jovius, the word for "Jupiter," the Roman god of the sky. Classical Latin Juppiter replaced Old Latin Jovis as the god's name.
The modern meaning comes from the astrological belief that people born under the planet Jupiter had good-humored or merry dispositions.