The true origin of yellow-belly and yellow-bellied is unknown. However, the term yellow was associated with cowardice since as far back as the 1850’s. It was most often seen in such expressions of yellow streak or streak of yellow. How yellow itself came to mean cowardly is unknown.

Yellow-bellied "cowardly" is from 1924, probably a semi-rhyming reduplication of yellow; earlier yellow-belly was a sailor's name for a half-caste (1867) and a Texas term for Mexican soldiers (1842, based on the color of their uniforms). Yellow dog "mongrel" is attested from c. 1770; slang sense of "contemptible person" first recorded 1881.