Why do we call a cowardly person “yellow”?

Yellow, meaning cowardly, is actually an abbreviation of yellow dog, an
American insult that first appeared in the nineteenth century to
describe a cowardly or worthless person. In the early twentieth centu-
ry, when employers were fighting trade unions, they insisted that new
employees sign a pledge never to join a union. This pledge was called
a “yellow dog” contract by union members with the implication that
anyone signing it was “yellow.”