Mare's-tail
Mare's-tail , noun
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.
2.
(Botany) An aquatic plant of the genus Hippuris (Hippuris vulgaris), having narrow leaves in whorls.