The Collaborative International Dictionary of English v.0.48:

Stag \Stag\ (st[a^]g), n. [Icel. steggr the male of several
   animals; or a doubtful AS. stagga. Cf. Steg.]
   1. (Zool.)
      (a) The adult male of the red deer (Cervus elaphus), a
          large European species closely related to the American
          elk, or wapiti.
      (b) The male of certain other species of large deer.
          [1913 Webster]

   2. A colt, or filly; also, a romping girl. [Prov. Eng.]
      [1913 Webster]

   3. A castrated bull; -- called also bull stag, and bull
      seg. See the Note under Ox.
      [1913 Webster]

   4. (Stock Exchange)
      (a) An outside irregular dealer in stocks, who is not a
          member of the exchange. [Cant]
      (b) One who applies for the allotment of shares in new
          projects, with a view to sell immediately at a
          premium, and not to hold the stock. [Cant]
          [1913 Webster]

   5. (Zool.) The European wren. [Prov. Eng.]
      [1913 Webster]

   Stag beetle (Zool.), any one of numerous species of
      lamellicorn beetles belonging to Lucanus and allied
      genera, especially Lucanus cervus of Europe and Lucanus
      dama of the United States. The mandibles are large and
      branched, or forked, whence the name. The larva feeds on
      the rotten wood of dead trees. Called also horned bug,
      and horse beetle.

   Stag dance, a dance by men only. [Slang, U.S.]

   Stag hog (Zool.), the babiroussa.

   Stag-horn coral (Zool.), any one of several species of
      large branching corals of the genus Madrepora, which
      somewhat resemble the antlers of the stag, especially
      Madrepora cervicornis, and Madrepora palmata, of
      Florida and the West Indies.

   Stag-horn fern (Bot.), an Australian and West African fern
      (Platycerium alcicorne) having the large fronds branched
      like a stag's horns; also, any species of the same genus.


   Stag-horn sumac (Bot.), a common American shrub (Rhus
      typhina) having densely velvety branchlets. See Sumac.


   Stag party, a party consisting of men only. [Slang, U. S.]


   Stag tick (Zool.), a parasitic dipterous insect of the
      family Hippoboscidae, which lives upon the stag and is
      usually wingless. The same species lives also upon the
      European grouse, but in that case has wings.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stag party
    n 1: a party for men only (or one considered suitable for men
         only) [syn: stag party, smoker]