show the white feather

show the white feather (display cowardice) — бояться, трусить; испугаться, проявить трусость; уйти в кусты; спрятаться в кусты (перен)

Example: The minute Bob put up his fists, Bill showed the white feather and backed down.

see also
[chicken out]
[get cold feet]
[throw in the towel]
[yellow-bellied]
[наложить в штаны]
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This expression comes from cockfighting, where a white feather in a bird's tail is considered a sign of inferior breeding. (Answers.com)