The tradition of giving white feathers to people who do not enlist with the military, despite appearing to be eligible, is hundreds of years old. The idea comes from the sport of cockfighting, in which roosters with a single white feather in their tails are believed to be poor fighters. The feather also indicates that a bird is a mongrel, rather than a purebred. When the practice of handing out such feathers was widespread, receiving a feather was supposed to indicate that someone was a coward.
Britain is most closely associated with the convention of the white feather, and this became especially true during the First World War. In 1914, Admiral Charles Fitzgerald started the Order of the White Feather, and encouraged women to give out such feathers to men out of uniform. This practice spread to Australia and the United States, and was designed to shame men who were not fighting. Some men heavily criticized the practice, arguing that since women were not eligible to serve, it was not reasonable for them to get involved in military politics.
Especially in Britain, someone may be said to be "showing the white feather" when they engage in an act of extreme cowardice. This concept is used in the civilian world, not just the military, although the intent of criticism is the same.
The politics of the white feather are complex. In some cases, a man may be out of uniform because he is serving his country in another capacity. Civil servants, doctors, and people in similar capacities were sometimes erroneously handed white feathers by people who did not understand why they were out of uniform. In Britain, people who were in this position were sometimes given badges to wear, so that they would not be “feathered” while going about their daily business. In other cases, a man might have been rejected for service due to ill health, and the feather would have further distressed and shamed him.
However, another group of men were actually proud to receive white feathers. The First World War saw an explosion of conscientious objectors, sometimes from within the military itself. These pacifists could not in good conscience support the war, and they received numerous feathers as a result. Several famous leaders within the movement were said to be delighted by receiving “enough feathers to make a fan.” The peace movement took the idea one step further in the 1930s and 1940s by actively adopting it as a symbol, although it is unrelated to the white dove of peace.