Definition of mare's tail
1a : a common aquatic plant (Hippuris vulgaris of the family Hippuridaceae) with elongated shoots having dense whorls of narrow finely tapered leavesb : horseweed 1
2 : a cirrus cloud that has a long slender flowing appearance
First Known Use of mare's tail
Seen and Heard
What made you want to look up mare's tail? Please tell us where you read or heard it (including the quote, if possible).