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. [1913 Webster] 2. (Brewing) A fermenting vat. [1913 Webster] 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. [1913 Webster] 4. (Com.) A weight of 2,240 pounds. See {Ton}. [R.] [1913 Webster] 5. An indefinite large quantity. --Shak. [1913 Webster] A tun of man in thy large bulk is writ. --Dryden. [1913 Webster] 6. A drunkard; -- so called humorously, or in contempt. [1913 Webster] 7. (Zo["o]l.) Any shell belonging to Dolium and allied genera; -- called also {tun-shell}. [1913 Webster]