1The tail of a mouse.
2A dwarf plant of seasonally wet places, Myosurus minimus (family Ranunculaceae), with single tiny yellowish-green flowers and numerous achenes in an elongating spike, which is native to Old World temperate regions and naturalized elsewhere. Also (with distinguishing word): any of several related North American species of Myosurus.
3 rare Any of certain other plants with a long slender inflorescence. In full "mousetail grass". A foxtail grass with a slender spike, especially black grass, Alopecurus myosuroides. Also (British regional) in plural with singular agreement.
4A fescue grass of the genus Vulpia having a narrow panicle; specifically (more fully "mousetail fescue" (also "mousetail fescue-grass"), † "mousetail grass"), rat's-tail fescue, V. myuros.
5 rare In singular and plural. US, slang. A moustache; specifically a pencil-line moustache.
Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. From mouse + tail.