The Collaborative International Dictionary of English v.0.48:
Thrasher \Thrash"er\, Thresher \Thresh"er\, n.
1. One who, or that which, thrashes grain; a thrashing
2. (Zool.) A large and voracious shark (Alopias vulpes),
remarkable for the great length of the upper lobe of its
tail, with which it beats, or thrashes, its prey. It is
found both upon the American and the European coasts.
Called also fox shark, sea ape, sea fox, slasher,
swingle-tail, and thrasher shark.
3. (Zool.) A name given to the brown thrush and other allied
species. See Brown thrush.
Sage thrasher. (Zool.) See under Sage.
Thrasher whale (Zool.), the common killer of the Atlantic.