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.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "thresh":
   bang, baste, batter, beat, bolt, buffet, cordon, cordon off,
   cull out, divide, drub, flail, flap, ghettoize, gin, hammer,
   insulate, isolate, keep apart, keep aside, knock, lambaste, larrup,
   lay aside, maul, paste, patter, pelt, pick out, pommel, pound,
   pulverize, pummel, put aside, quarantine, rap, riddle, screen,
   seclude, segregate, separate, set apart, set aside, sieve, sift,
   sledgehammer, sort out, spank, thrash, thump, wallop, whip,
   winnow