Leather shields placed beside a horse's eyes to limit vision and keep it focused forward; also means something that prevents understanding.
From 'blind,' which comes from Old English 'blind.' The horse equipment meaning emerged in the 16th century, while the figurative 'preventing understanding' meaning developed later as a metaphor.
Horses wearing blinders inspired the phrase 'to have blinders on,' which means being unwilling to see the truth—it's such a perfect metaphor that we use it constantly without even thinking about the actual horse equipment!
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