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
      machine.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Zool.) A name given to the brown thrush and other allied
      species. See Brown thrush.
      [1913 Webster]

   Sage thrasher. (Zool.) See under Sage.

   Thrasher whale (Zool.), the common killer of the Atlantic.
      [1913 Webster]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thresher \Thresh"er\, n.
   Same as Thrasher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thresher
    n 1: a farm machine for separating seeds or grain from the husks
         and straw [syn: thresher, thrasher, threshing
         machine]
    2: large pelagic shark of warm seas with a whiplike tail used to
       round up small fish on which to feed [syn: thresher,
       thrasher, thresher shark, fox shark, Alopius
       vulpinus]