The compound adjective "yellow-bellied" originated in the early 19th century as a literal descriptor for animals, such as birds or snakes, with yellowish underparts. Its figurative sense of "cowardly" emerged in American English around the 1920s, likely influenced by the long-standing association of the color yellow with concepts like jealousy, sickness, and treachery, but more directly from the idea that a coward's liver was supposedly lacking blood or was "white-livered." The logic of the insult transfers a visible physical trait from the animal kingdom to imply a lack of moral or courageous fortitude in a person.

Structure: yellow (color) + bellied (having a belly) - yellow: From Old English geolu, geolwe, of Germanic origin, related to gold. Denotes the color. - bellied: From Old English belg (bag, pouch, belly). The suffix -ed forms an adjective meaning "having a [specified kind of] belly."