a brass ornament, originally intended for the harness of a horse.
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British Dictionary definitions for horse brass
a decorative brass ornament, usually circular, originally attached to a horse's harness
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012