Source : Webster's Revised Unabridged Dictionary (1913)

Mare's-tail \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} ({H.
      vulgaris}), having narrow leaves in whorls.