The Collaborative International Dictionary of English v.0.48:

Flail \Flail\, n. [L. flagellum whip, scourge, in LL., a
   threshing flail: cf. OF. flael, flaiel, F. fl['e]au. See
   Flagellum.]
   1. An instrument for threshing or beating grain from the ear
      by hand, consisting of a wooden staff or handle, at the
      end of which a stouter and shorter pole or club, called a
      swipe, is so hung as to swing freely.
      [1913 Webster]

            His shadowy flail hath threshed the corn. --Milton.
      [1913 Webster]

   2. An ancient military weapon, like the common flail, often
      having the striking part armed with rows of spikes, or
      loaded. --Fairholt.
      [1913 Webster]

            No citizen thought himself safe unless he carried
            under his coat a small flail, loaded with lead, to
            brain the Popish assassins.           --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flail
    n 1: an implement consisting of handle with a free swinging
         stick at the end; used in manual threshing
    v 1: give a thrashing to; beat hard [syn: thrash, thresh,
         lam, flail]
    2: move like a flail; thresh about; "Her arms were flailing"
       [syn: flail, thresh]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "flail":
   bang, baste, bastinado, batter, beat, belabor, belt, birch, buffet,
   cane, club, cowhide, cudgel, cut, cut and thrust, drub, feint,
   flagellate, flail at, flail away at, flap, flog, fustigate,
   give a whipping, give the stick, hammer, hit at, horsewhip, knock,
   knout, lace, lambaste, larrup, lash, lash out at, lay on,
   let drive at, let fly at, lunge at, maul, paste, patter, pelt,
   pistol-whip, poke at, pommel, pound, pulverize, pummel, rap,
   rawhide, scourge, sledgehammer, smite, spank, strap, strike at,
   strike out at, stripe, swing at, swing on, swinge, switch, thrash,
   thresh, thrust at, thump, trounce, truncheon, wallop, whale, whip,
   whop