1580s, "under the influence of the planet Jupiter," from Middle French jovial (16c.), from Italian joviale, literally "pertaining to Jupiter," and directly from Late Latin Iovialis "of Jupiter," from Latin Iovius (used as genitive of Iuppiter) "of or pertaining to Jupiter," Roman god of the sky (see Jove). The meaning "good-humored, merry," is from the astrological belief that those born under the sign of the planet Jupiter are of such dispositions. Related: Jovially.