Mare's-tail /(?)/

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. An aquatic plant of the genus Hippuris (Hippuris vulgaris), having narrow leaves in whorls. (Bot.)