To show the white feather

Significa demostrar cobardía o temor, retirarse. If a boxer shows the white feather, he must give up boxing: Si un boxeador demuestra cobardía, debe abandonar el boxeo.

Dictionary.com nos cuenta un poco el origen de esta expresión:

Origin: 1775–85; orig. from a white feather in a gamecock's tail, taken as a sign of inferior breeding and hence of poor fighting qualities.

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