"Gibbous" means humpbacked. English has no everyday word for the shape, so astronomy borrowed a medical one.

From the Latin gibbus, a hump. It described hunched posture in medieval medical writing long before it described the sky. Astronomers use it for Mars and Venus too, which show phases of their own. Most languages have a word for crescent and nothing at all for this, which may be part of why so little has ever been written about it.