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]

WordNet (r) 3.0 (2006):

thrash
    n 1: a swimming kick used while treading water
    v 1: give a thrashing to; beat hard [syn: thrash, thresh,
         lam, flail]
    2: move or stir about violently; "The feverish patient thrashed
       around in his bed" [syn: convulse, thresh, thresh
       about, thrash, thrash about, slash, toss,
       jactitate]
    3: dance the slam dance [syn: slam dance, slam, mosh,
       thrash]
    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: thrash, thresh]
    7: beat thoroughly and conclusively in a competition or fight;
       "We licked the other team on Sunday!" [syn: cream, bat,
       clobber, drub, thrash, lick]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "thrash":
   bang, baste, bastinado, batter, beat, beat all hollow, beat hollow,
   belabor, belt, best, birch, bolt, buffet, cane, clobber, club,
   cordon, cordon off, cowhide, cudgel, cull out, cut, defeat,
   destroy, divide, do in, drub, fix, flagellate, flail, flap, flog,
   fustigate, ghettoize, gin, give a whipping, give the stick, hammer,
   hide, hors de combat, horsewhip, insulate, isolate, keep apart,
   keep aside, knock, knout, lace, lambaste, larrup, lash, lather,
   lay aside, lay on, lick, maul, outclass, outdo, outfight,
   outgeneral, outmaneuver, outpoint, outrun, outsail, outshine, pail,
   paste, patter, pelt, pick out, pistol-whip, pommel, pound,
   pulverize, pummel, put, put aside, quarantine, rap, rawhide,
   riddle, ruin, scourge, screen, seclude, segregate, separate,
   set apart, set aside, settle, shellac, sieve, sift, skin,
   skin alive, sledgehammer, smear, smite, smother, sort out, spank,
   strap, strike, stripe, swinge, switch, thresh, thump, trim,
   triumph over, trounce, truncheon, undo, wallop, wear out, whale,
   whip, whop, winnow, worst



The Jargon File (version 4.4.7, 29 Dec 2003):

thrash
 vi.

    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. Someone who keeps changing his mind (esp.
    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.


The Free On-line Dictionary of Computing (30 December 2018):

thrash
thrashing

   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]