Source : Webster's Revised Unabridged Dictionary (1913)

Thrash \Thrash\, Thresh \Thresh\, v. t. [imp. & p. p.
   {Thrashed}; p. pr. & vb. n. {Thrashing}.] [OE.
   [thorn]reschen, [thorn]reshen, to beat, AS. [thorn]erscan,
   [thorn]rescan; akin to D. dorschen, OD. derschen, G.
   dreschen, OHG. dreskan, Icel. [thorn]reskja, Sw. tr["o]ska,
   Dan. t[ae]rske, Goth. [thorn]riskan, Lith. traszketi to
   rattle, Russ. treskate to burst, crackle, tresk' a crash,
   OSlav. troska a stroke of lighting. Cf. {Thresh}.]
   1. To beat out grain from, as straw or husks; to beat the
      straw or husk of (grain) with a flail; to beat off, as the
      kernels of grain; as, to thrash wheat, rye, or oats; to
      thrash over the old straw.

            The wheat was reaped, thrashed, and winnowed by
            machines.                             --H. Spencer.

   2. To beat soundly, as with a stick or whip; to drub.

Thrash \Thrash\, Thresh \Thresh\, v. t.
   1. To practice thrashing grain or the like; to perform the
      business of beating grain from straw; as, a man who
      thrashes well.

   2. Hence, to labor; to toil; also, to move violently.

            I rather would be M[ae]vius, thrash for rhymes, Like
            his, the scorn and scandal of the times. --Dryden.

Thresh \Thresh\, v. t. & i. [imp. & p. p. {Threshed}; p. pr. &
   vb. n. {Threshing}.]
   Same as {Thrash}.

         He would thresh, and thereto dike and delve. --Chaucer.

Source : WordNet®

thresh
     v 1: move or stir about violently; "The feverish patient thrashed
          around in his bed" [syn: {convulse}, {thresh about}, {thrash},
           {thrash about}, {slash}, {toss}, {jactitate}]
     2: move like a flail; thresh about; "Her arms were flailing"
        [syn: {flail}]
     3: beat the seeds out of a grain [syn: {thrash}]
     4: give a thrashing to; beat hard [syn: {thrash}, {lam}, {flail}]