The Collaborative International Dictionary of English v.0.48:

Tun \Tun\ (t[u^]n), 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.
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   2. (Brewing) A fermenting vat.
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   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.
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   4. (Com.) A weight of 2,240 pounds. See Ton. [R.]
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   5. An indefinite large quantity. --Shak.
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            A tun of man in thy large bulk is writ. --Dryden.
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   6. A drunkard; -- so called humorously, or in contempt.
                                                  --Dryden.
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   7. (Zool.) Any shell belonging to Dolium and allied genera;
      -- called also tun-shell.
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The Collaborative International Dictionary of English v.0.48:

Tun \Tun\, v. t. [imp. & p. p. Tunned; p. pr. & vb. n.
   Tunning.]
   To put into tuns, or casks. --Boyle.
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WordNet (r) 3.0 (2006):

tun
    n 1: a large cask especially one holding a volume equivalent to
         2 butts or 252 gals

Bouvier's Law Dictionary, Revised 6th Ed (1856):

TUN, measure. A vessel of wine or oil, containing four hogsheads.