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.

Source : WordNet®

thrash
     n : a swimming kick used while treading water

thrash
     v 1: give a thrashing to; beat hard [syn: {thresh}, {lam}, {flail}]
     2: move or stir about violently; "The feverish patient thrashed
        around in his bed" [syn: {convulse}, {thresh}, {thresh
        about}, {thrash about}, {slash}, {toss}, {jactitate}]
     3: dance the slam dance [syn: {slam dance}, {slam}, {mosh}]
     4: beat so fast that (the heart's) output starts dropping until
        (it) does not manage to pump out blood at all
     5: move data into and out of core rather than performing useful
        computation; "The system is thrashing again!"
     6: beat the seeds out of a grain [syn: {thresh}]
     7: beat thoroughly in a competition or fight; "We licked the
        other team on Sunday!" [syn: {bat}, {clobber}, {drub}, {lick}]

Source : Free On-Line Dictionary of Computing

thrash
     
        To move wildly or violently, without accomplishing anything
        useful.  {Paging} or {swapping} systems that are overloaded
        waste most of their time moving data into and out of {core}
        (rather than performing useful computation) and are therefore
        said to thrash.  Thrashing can also occur in a {cache} due to
        {cache conflict} or in a {multiprocessor} (see {ping-pong}).
     
        Someone who keeps changing his mind (especially about what to
        work on next) is said to be thrashing.  A person frantically
        trying to execute too many tasks at once (and not spending
        enough time on any single task) may also be described as
        thrashing.
     
        Compare {multitask}.
     
        [{Jargon File}]