The Collaborative International Dictionary
Mare's-tail \Mare's"-tail`\, n.
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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. (Bot.) An aquatic plant of the genus Hippuris ( Hippuris vulgaris), having narrow leaves in whorls.
Wiktionary
n. 1 A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. 2 An aquatic plant, (taxlink Hippuris vulgaris species noshow=1), having narrow leaves in whorls.
Usage examples of "mare's-tail".
On the porch of an old frame house the woman sits in an arrow-back chair as the first heavy drops hit the street, raising dust in gauzy mare's-tails.
A brooch shaped like the Labrys upheld a cloak whose lining was white as a summer cloud but which outside was grey for thunderheads and mare's-tails.