The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            The wheat was reaped, thrashed, and winnowed by
            machines.                             --H. Spencer.
      [1913 Webster]

   2. To beat soundly, as with a stick or whip; to drub.
      [1913 Webster] Thrash

The Collaborative International Dictionary of English v.0.48:

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.
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   2. Hence, to labor; to toil; also, to move violently.
      [1913 Webster]

            I rather would be Maevius, thrash for rhymes,
            Like his, the scorn and scandal of the times.
                                                  --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thresh \Thresh\, v. t. & i. [imp. & p. p. Threshed; p. pr. &
   vb. n. Threshing.]
   Same as Thrash.
   [1913 Webster]

         He would thresh, and thereto dike and delve. --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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