Here's what Wikipedia says about white feathers:

The white feather is a widely recognised propaganda symbol It has, among other things, represented cowardice or conscientious pacifism; as in A. E. W. Mason's 1902 book, The Four Feathers. In Britain during the First World War it was often given to men out of uniform by women to shame them publicly into signing up. In the United States armed forces, however, it is used to signify extraordinary bravery and excellence in combat marksmanship.

As a symbol of cowardice
In the United Kingdom and the countries of the British Empire since the 18th century, the white feather has sometimes represented cowardice. As such, it was used by patriotic groups, including prominent members of the Suffragette movement and early feminists, to shame men into enlisting.

Accordingly, the white feather supposedly comes from cockfighting and the belief that a cockerel sporting a white feather in its tail is likely to be a poor fighter. Pure-breed gamecocks do not show white feathers and so its presence indicates that the cockerel is an inferior cross-breed.

The Crusades
Shame was exerted upon men in England and France who had not taken the cross at the time of the Third Crusade. "A great many men sent each other wool and distaff, hinting that if anyone failed to join this military undertaking they were only fit for women's work".

World War I
In August 1914, at the start of World War I, Admiral Charles Fitzgerald founded the Order of the White Feather with the support of the prominent author Mary Augusta Ward. The organisation aimed to shame men into enlisting in the British army by persuading women to present them with a white feather if they were not wearing a uniform.[4][5]

Although the draft would conscript both sexes, only males would be on the front lines. While the true effectiveness of the campaign is impossible to judge, it spread throughout several other nations in the empire. In Britain, it started to cause problems for the government when public servants and men in essential occupations came under pressure to enlist. That prompted Home Secretary Reginald McKenna to issue employees in state industries with lapel badges reading "King and Country" to indicate that they were serving the war effort. Likewise, the Silver War Badge, which was given to service personnel who had been honourably discharged by wounds or sickness, was first issued in September 1916 to prevent veterans from being challenged for not wearing uniform. Anecdotes from the time indicate that the campaign was not popular among soldiers, not least because soldiers who were home on leave could find themselves presented with feathers.

One example was Private Ernest Atkins, who was on leave from the Western Front. He was riding a tram when he was presented with a white feather by a girl sitting behind him. He smacked her across the face with his pay book and said, "Certainly I'll take your feather back to the boys at Passchendaele. I'm in civvies because people think my uniform might be lousy, but if I had it on I wouldn't be half as lousy as you".

Private Norman Demuth, who had been discharged from the British Army after he had been wounded in 1916, received numerous white feathers after he returned from the Western Front. In Forgotten Voices of the Great War, Demuth is quoted as saying:

"Almost the last feather I received was on a bus. I was sitting near the door when I became aware of two women on the other side talking at me, and I thought to myself, 'Oh Lord, here we go again'. One lent forward and produced a feather and said, 'Here's a gift for a brave soldier. I took it and said,'Thank you very much- I wanted one of those.' Then I took my pipe out of my pocket and put this feather down the stem and worked it in a way I've never worked a pipe cleaner before. When it was filthy I pulled it out and said, 'You know, we didn't get these in the trenches', and handed it back to her. She instinctively put out her hand and took it, so there she was sitting with this filthy pipe cleaner in her hand and all the other people on the bus began to get indignant. Then she dropped it and got up to get out, but we were nowhere near a stopping place and the bus went on quite a long way while she got well and truly barracked by the rest of the people on the bus. I sat back and laughed like mad."

Supporters of the campaign were not easily put off. A woman who confronted a young man in a London park demanded to know why he was not in the army. "Because I am a German", he replied. He received a white feather anyway.

Perhaps the most misplaced use of a white feather was when one was presented to Seaman George Samson, who was on his way in civilian clothes to a public reception in his honour. Samson had been awarded the Victoria Cross for gallantry in the Gallipoli campaign.

Roland Gwynne, later the mayor of Eastbourne (1929–1931) and a lover of suspected serial killer John Bodkin Adams, received a feather from a relative. That prompted him to enlist, and he would receive the Distinguished Service Order for bravery.

The writer Compton Mackenzie, then[when?] a serving soldier, complained about the activities of the Order of the White Feather. He argued that "idiotic young women were using white feathers to get rid of boyfriends of whom they were tired". The pacifist Fenner Brockway said he received so many white feathers that he had enough to make a fan.

World War II
The white feather campaign was briefly renewed during World War II.

Etc., etc.