show the white feather - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Display cowardice, as in The minute Bob put up his fists, Bill showed the white feather and backed down. This expression comes from cockfighting, where a white feather in a bird's tail is considered a sign of inferior breeding. [Early 1800s]