gibbous adj.

c. 1400, "bulging, convex," from Late Latin gibbus "hunchbacked," from Latin gibbus "a hump, a hunch," as an adjective, "bulging," from Proto-Italic *gifri- "hump," *gifro- "hump-backed," of uncertain origin. De Vaan suggests a PIE *geibh-, with possible cognates in Lithuanian geibus "gawky, plump," geibstu, geibti "become weak;" Norwegian dialect keiv "slanted, wrong," keiva "left hand," perhaps united by a general sense of "bodily defect." Of the moon from early 15c.; also used from 15c. of hunchbacks. Related: Gibbosity.