Source : Webster's Revised Unabridged Dictionary (1913)

Tun \Tun\, v. i. [imp. & p. p. {Tunned}; p. pr. & vb. n.
   {Tunning}.]
   To put into tuns, or casks. --Boyle.

Tun \Tun\, n. [AS. tunne. See {Ton} a weight.]
   1. A large cask; an oblong vessel bulging in the middle, like
      a pipe or puncheon, and girt with hoops; a wine cask.

   2. (Brewing) A fermenting vat.

   3. A certain measure for liquids, as for wine, equal to two
      pipes, four hogsheads, or 252 gallons. In different
      countries, the tun differs in quantity.

   4. (Com.) A weight of 2,240 pounds. See {Ton}. [R.]

   5. An indefinite large quantity. --Shak.

            A tun of man in thy large bulk is writ. --Dryden.

   6. A drunkard; -- so called humorously, or in contempt.

   7. (Zo["o]l.) Any shell belonging to Dolium and allied
      genera; -- called also {tun-shell}.

Source : WordNet®

tun
     n : a large cask especially one holding a volume equivalent to 2
         butts or 252 gals
     [also: {tunning}, {tunned}]