A U-shaped metal shoe nailed to a horse's hoof for protection, or something shaped like this that is considered lucky.
From Old English combining 'horse' and 'shoe.' Iron horseshoes became common in medieval Europe (around the 9th century) as horses increasingly needed protection for heavy work. The lucky superstition likely arose because iron was precious and a horseshoe represented wealth.
Horseshoes might be lucky because of pure economics—losing a horseshoe meant expensive damage to your horse, so finding one was genuinely fortunate! Medieval people turned this practical concern into magical superstition, and the superstition stuck even though nobody needs horseshoes anymore.
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