Though other planetary bodies can appear gibbous in our skies, the word is for excellent reasons most closely attached to the moon. It’s said to be gibbous when it’s more than halfway towards full, when the crescent of its early days has filled out to make a convex shape, and also while it wanes but is still more than half full.
The link is so close that it comes as a surprise to find that it has other meanings and that, indeed, its application to the moon is a bit of a Johnny-come-lately among its senses. The origin is Latin gibbus, a hump, and its first meaning in English was of something rounded or protuberant. The medieval Italian surgeon Lanfranc of Milan wrote, in modern English translation, “On one side he is gibbous but on the other side he is flatter.” Many of us are that shape, especially us older ones.
Gibbous has been used in medicine to describe tumours and other deformities and in biology for bones, fruits, leaves, shells or fungi. Wider still it could be anything convex, including rocks, jars or even habitations — Walter Pater wrote of “the range of old gibbous towns ... expanding their gay quays upon the water-side.”
However, it’s now largely restricted to astronomical bodies. This is a pity, as it’s a fine word that deserves to be more used. So I was delighted to come across a description of the well-muscled popstress Madonna in the Daily Telegraph some years ago: “She was photographed last week with veins popping on her gibbous biceps as she strode out of a restaurant.”