Mare's-tail n.
  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. Bot. An aquatic plant of the genus Hippuris (Hippuris vulgaris), having narrow leaves in whorls.
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