c.1400, "bulging, convex," from Late Latin gibbus "hunchbacked," from Latin gibbus "hump, hunch," of uncertain origin. Of the moon from early 15c.; also used from 15c. of hunchbacks.
gibbous gib·bous (gĭb'əs)
Characterized by convexity; protuberant.
Having a hump; humpbacked.
More than half but less than fully illuminated. Used to describe the Moon or a planet. Compare crescent.