Yellow or yellow-belly meaning cowardly is of American origin from the mid-19th century. It is thought that the expression yellow-belly may have been first applied to Mexicans c.1842, but whether it referred to Mexican army uniforms of the era, which used yellow, or to the Mexican complexion or skin colour, is not clear. A person having a yellow streak or cowardly tendencies is also American from the early 20th century. Quite why yellow should be associated with cowardice remains unknown. However, there might be more to the colour yellow than meets the eye. It may have all started with the four humours on which Greek medicine and indeed much of European and Arabic medicine is based. According to Hippocrates, basic human moods or humours are caused by lack or excess of certain bodily fluids. One of the four humours was yellow bile which made people more or less choleric, which was not a very auspicious association for the colour yellow. In Europe, the colour yellow has long been associated with treachery. Brewster’s informs us that in France the doors of suspected traitors were daubed with yellow. The medieval yellow star, resurrected by the Nazis, branded the Jews as betrayers of Christ. In medieval paintings, Judas Iscariot, the ultimate betrayer, frequently appears dressed in yellow garments. Victims of the Spanish Inquisition were forced to wear yellow garments to symbolise their heresy. For many years, the dreaded yellow flag signalled to other ships that there was a contagious disease like cholera aboard. As ignoble as some of these examples are, there seems to be some dark, unpleasant connotation attached to the colour yellow. So why not attach it to cowardice? See also Bad blood.