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]