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