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Medical Definition of Mare's-tail
1. 
1. A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. See Cloud. "Mackerel sky and mare's-tails Make tall ships carry low sails." (Old Rhyme)
2. 
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1. 
1. A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. See Cloud. "Mackerel sky and mare's-tails Make tall ships carry low sails." (Old Rhyme)
2.